A series of laboratory cone penetration tests using a calibration chamber are conducted on different types of cohesive soil to study the effect of various factors such as plasticity index, rigidity index and strain rate on cone resistance. It is found out that cone resistance increases with penetration rates. An attempt is made to predict cone resistance theoretically by incorporating the strain rate effect into the theory of cavity expansion. The theory compares well with the observation. It is shown that the effect of strain rate is very similar to that of rigidity index.