Harmonic generation in BBO by femtosecond laser pulses is studied, taking into account both group velocity mismatch, lowest order group velocity dispersion (GVD) and second order GVD. The lowest and second order GVD of BBO as a function of wavelength is calculated. Second and third harmonic radiation with femtosecond laser pulses is computed by numerically solving the improved coupled wave equations. The effects of the lowest oder GVD and second order GVD on fundamental and harmonic pulses are analyzed. The compensation of group velocity mismatch for third harmonic generaiton is considered.