By solving numerically the onedimensional timedependent Schrdinger equation, the highorder harmonic generation of using ultrashort laser pulses with three pulse durations(10,20 and 40 optical cycles) is studied. It is showed that the harmonic spectrum of our model atom with 10-cycle laser pulses contains higherorder harmonic than that with 20 and 40- cycle laser pulses. It is conculded that the increase in harmonic order with shorter laser pulse is due to the weaker ionization of the atom, i.e., the atom can experience higher laser intensities before it is ionized with shorter laser pulses than that with longer laser pulses.