Using properly shaped metal forms, the reinforced concrete slabs with grid-beams, call 'PANSLAB' hereafter, are constructed easily. And, this grid-slab is expected much stronger and more economical as compared with the conventional floor slab. Those superiority of the structure were examined experimentally by the full-size modeltests. Panslabs, with 12.5cm×25cm section grid-beams of 1m bay and with 5cm plain concrete slab on their upper surface, were compared with 11cm thick usual slabs about their strength and rigidity. Both were made of nearly the same amount of concrete, and were designed according to the same Japanese conventional design criteria under the same design condition. To examine the effect of the slab of a panslab, pure grid-beams were also tested for comparison. Model size were 4m×4m and 4m×5m, and in each group were tested three panslabs, one usual slab and one pure grid-beams. Each edge of the testpieces was framed into a rigid beam respectively so as to make it nearly in a clamped state. Testpieces were supported on 4 balls by the 4 corners and the equal concentrated loads were applied at each joint of the cross-beams. In this report we examined about the ultimate strength of the test-pieces experimentally and theoretically. The result was as follows: 1. At the design load, both panslabs and usual slabs do not show any defect about strength, rigidity and crack development. In the conventional design method, the steel used in a panslab is much less than than in a usual slab under the same design load. 2. Panslads are much stronger than usual slabs because of the tension resistance of the plain concrete slab. From this result, a more economical design method is suggested for panslabs. For these tests, the safety factor of the design load against the yield load of the panslabs was 5〜7 and this was more than 1.4 times the value of the usual slabs. 3. The slab failure of a panslab occures suddenly in a brittle state and after the yielding the bearing load of a panslab decreases to some stable value and then the panslab preserves a plastic tenacity, so we infer that it is reasonable to choose this plastic state as a standard of effective yielding point. The effective yield load of the panslabs in these tests were 0.7〜0.8 times the yield load. Again, for the same reason we should place a proper reinforcement in the upper slab of a panslab in case an excessive tension stress concentration would take place in the slab. 4. Effect of the temperature stress and shrinkage stress in the upper slab and the shear charcteristics of a panslab are not examined in these tests.