In this paper, nonlinear frequency conversion of second harmonic and third harmonic of femtosecond Ti:sapphire laser is theoretically and experimentally studied. Noncolinear type Ⅰ phase matching process is chosen to generate third harmoni c of femtosecond Ti:sapphire laser, and we have obtained conversion efficiency o f 62% at 260nm output. The pulse duration is about 169fs. This work will s erve as femtosecond ultraviolet source for time_resolved electron microscopy.