The conclusion that the pulse-width is proportional to the square root of the third order dispersion coefficient is obtained by using the modified higher-order nonlinear Schr?dinger equation with concurrent actions of third order dispersion and self-steepen effect when a femtosecond pulse propagates steadily in a laser. This result is in agreement with the experimental result. Besides, the action of second order dispersion term is also discussed in the realization of mode locking process of a laser.