During the loading, a high precision measurement of pore water pressure is helpful to understand explicitly the rule of changes of pore water pressure in clays, and plays an important role in instructing the engineering practices. The Cambridge self-boring expansion pressuremeter manufactured by Cambridge Insitu Companly in British is an advanced site investigation device. By means of tests, the instrument provides accurate information about pore water pressures during pressing. Two different types of clays are tested with the self-boring pressuremeter, and in situ data about pore water pressure are analyzed. The test results show that during the loading phase, an effective stress in clays increases monotonically, while pore water pressure generated in creases at the starting, and then decreases. Beside that, the amplitude of pore water pressure accumulated in a stiff clay is lower than at in a soft clay; however, the absolute value of pore water pressure of former is further higher than that of the latter, which indicates that the strength and stiffness of clays are important influence factors to the changes of pore water pressure.