Pulse compression by sum frequency generation is demonstrated in femtosecond time domain with a type II phase matching BBO crystal. By frequency mixing in BBO crystal infrared signal and idler pulses of 160 fs duration generated by an optical parametric amplifier, sum frequency pulses as short as 34 fs were obtained. Experiments and computer simulations demonstrate the potential of this technique to produce ultrashort pulses with high conversion efficiency. ©1996 American Institute of Physics.