In this work, the influence of the length-to-diameter ratio (L/D) on the unconfined compressive strength (UCS) of cohesive soil specimens was assessed. An L/D ratio of between 2 and 3 is generally recommended by accepted scientific and technical authorities. Nevertheless, published reports on the effect of specimen shape on soil strength are scarce. Therefore, we determined the effect of specimen shape on the UCS values of four clay soils by testing compacted cylindrical specimens with L/D ratios ranging from 0.5 to 3. The test results indicated that the UCS value decreases significantly with increasing L/D ratio. This decrease becomes particularly steep when the L/D ratio exceeds 1, but then becomes less steep when the L/D ratio is between 1.25 and 2.5. In addition, the failure pattern generally changes from ductile to brittle and the failure mechanism develops in quite complex and obscure ways when the L/D ratio is a parts per thousand yen2.75. Based on an analysis of the results, correction formulae with strong statistical relationships were determined for the soils tested in this study. Nonetheless, further investigation is needed to check the validity of the equations derived for these soils.