Handsheets with different densities were prepared by TAPPI Standards, and expansivity and swelling stress of these sheets were measured. The effect of Interfiber space on the dimensional stability of papers was discussed. The results obtained are summarized below.<BR>1. Papers made from unbeaten or slightly beaten pulps containing a small amount of residual hemicellulose, such as linter pulp and alkali extracted LBKP, were rather poor in dimensional stability as compared with chemical pulps of paper making grade, but they did not increase in dimensional instability even when they were made from highly beaten pulps.<BR>2. On the contrary, papers made from chemical pulps of paper making grade greatly in-creased in dimensional instability with excessive beating of the pulp. Paper made from ground-wood pulp which contained a large amount of short fiber was also poor in dimensional stability.<BR>3. These pulps were roughly categorized into three groups according to their adsorptivities : linter pulp group, paper grade chemical pulp group and groundwood pulp group. Absorptivity depends on the chemical component of the pulps.<BR>4. For low density boards, moist-expansivity of the boards made from high C. S. freeness pulps as measured by thickness was larger than boards having the same density from low C. S. freeness pulps. But for high density boards, there were no appreciable differences in moist-expansivity among boards made from pulps of several degrees of C. S. freeness.<BR>5. A similar tendency was found in acid hydrolysis of pulps. That is, moist-expansivity of boards from unhydrolyzed pulp was larger than boards from acid hydrolyzed pulps for low density, but there were no appreciable differences in moist-expansivity among boards from these pulps for high density.<BR>6. The moist-expansivity of paper of low density was largely affected by the kind of pulp, but there was no significant difference at high densities.<BR>7. Swelling stresses due to absorbing water greatly correlated with the density of the boards and could be calculated by the following formula; <BR>y=a·x^b (x : density of board, y : swelling stress) <BR>Swelling stresses of several pulps were in the following order.<BR>hardwood BKP > linter pulp, softwood BKP > softwood GP > softwood BSP<BR>8. Swelling stress of pulp was guessed to be higher than 1.5 ton/g, pulp in the case of density of 1.5, so it was very difficult to make the paper free from moist- or wet-expansion. In conclusion, it is most effective to increase interfiber space in the paper in order to make paper having high dimensional stability.