The influence of group velocity dispersion (GVD) on the self-focusing of femtosecond laser pulses is investigated by numerically solving the extended nonlinear Schr?dinger equation. By introducing the GVD length LGVD into the semi-empirical, self-focusing formula proposed by Marburger, a revised one is proposed, which can not only well explain the influence of GVD on the collapse distance, but also is in good agreement with the numerical results, making the self-focusing formula applicable for more cases.