Representative length scale of ULSI (Ultra Large Scale Integration) cells is going to be at a nano-meter order, and the atomic level defects, such as uneven oxide films or dislocation accumulation are becoming more and more important. Among these defects, dislocation accumulation is known to be caused by thermo-plastic deformation in silicon during the processes of device fabrication. In this study, we analyse such thermal stress, plastic slip deformation and accumulation of dislocations in STI (Shallow Trench Isolation) type ULSI devices when the temperature drops from the initial at 1000 °C to room temperature. For the analysis, we use a crystal plasticity analysis code CLP, assuming that lattice friction stress for the movement of dislocations is proportional to the hardness of silicon, which is known to have strong dependency on temperature. The results show that dislocations are generated between the temperature range from 880 to 800 °C, and its maximum density is highly dependent on the lattice friction stress in the temperature range above 800 °C. For example, the difference of 16 MPa in the lattice friction stress at 1000 °C caused increase in dislocation density more than ten times. It is concluded that control of lattice friction stress at high temperatures is one of the most promising way for the suppression of dislocation accumulation.