Strained Si CMOS technology is one of the most advanced technologies in present day microelectronics. Electronical conductivity effective mass of strained Si is a key parameter to study electron mobility enhancement. Using K ·P method with the help of perturbation theory, dispersion relation near conduction band valley was determined, including the longitudinal and transverse masses. And then, electronical conductivity effective masses of strained Si on (001),(101) and (111) planes were obtained with respect to stress, splitting energy and directions. It was found that both the [100]and [010]directional electronical conductivity effective masses of strained Si/(001)Si1-xGex and the [010]directional one of strained Si/(101)Si1-xGex decrease with increasing Ge fraction or stress, and both values tend to be constant. The results provide valuable reference to the conduction channel design related to stress and orientation in the strained Si nMOS devices.