In Japan, domestic timber has about 50% of for post and sill, but has only 9% for beam or joist. MOE of Japanese cedar, the representative species of domesbtic timber, is lower than imported timber. (MOE of Japanese cedar is from 5 to 8GPa.) The cross-sectional size of floor beam is almost determined depend on bending deflection in practical design. Since using domestic timber for beam makes a result of increasing timber volume and spending much money, it is considered not to be used (or not to be produced). If the cost problem would be solved, there is possibility to increase demand of domestic timber for beam. We think use of higher efficiency of cross section, and aim at developing I-beam what is suitable for conventional structure in Japan. The goal of developing is to save 50% of timber volume for floor beam.<br> Upon selecting the main material, the web had a few options. It was determined by checking the shear property in the preliminary test, considering the connecting method and other factors. Structural test of the beam, durability test and long-term performance test are included in the procedure for developing I-beam, and finally design method for I-beam has been proposed.<br> I-joist products in US and Canada often has OSB for web. Because OSB has excellent shear property, and cost is also cheap. But there is no OSB made from domestic tree species, and also no OSB factory in Japan. Finally, plywood and Diagonal Lattice Panel (DLP) become options for web. DLP is a diagonal grid lattice panel (thickness: 18mm), that has 2ply of 45 degree diagonal laminas arranged at grid interval of 151mm. Cross-section size of the lamina is 55mm wide × 9mm thick, and those are bonded each other by using API adhesive.<br> Two Rail Shear Tests were carried out for finding out appropriate material from plywood and DLP. The results shows DLP has excellent shear property. Modulus of shear stiffness for DLP is about 2.2 times of that of Plywood, and 0.85 times of OSB. Comparing shear strength with OSB, average shear strength of OSB is 8.44MPa, that is 2 times of DLP. However, if simply compared as a single panel, since DLP (18mm thick) has twice thickness of OSB (9.5mm thick), Shear capacity is nearly equal and shear stiffness is 1.6 times of OSB. Two of them are almost same timber volume. DLP has goodness that pipes can pass through easily like water supply or cable ducts. We decided to use DLP for I-beam web, because of these advantages.<br> Designed I-beam has LVL flanges using Japanese larch and web of DLP. The flanges and the web are connected using tongue and groove joint of two rows with resorcinol adhesives. Beam tests were carried out, that include bending and shear test in some kind of different loading. These tests results established design strength and stiffness.<br> Shear strength decreasing was observed in the tests depend on shear zone length, length effect in shear was found. In order to study whether the length effect can be explained by the weakest link theory, we tried to predict another test value from the cumulative distribution function of strength given by Weibull distribution from observed shear test data. The deviation between the experimental strength and the predicted strength was so large that there might be something affecting length effect beside the weakest link theory. Effective second moment of area of section, effective modulus of section, shear stress coefficient in cross section were calculated by the equivalent homogeneous cross section method. These structural property for design and beam design method were proposed.