The interactions between two femtosecond dark solitons are numerically investigated. The results show that femtosecond dark solitons are less affected than bright solitons by their respective interaction forces. The latter attracts each other and collides into one pulse, then they keep the state of repulsion without periodicity. The former repulses all the way and the repulsive forces are weaker than those of bright solitons. They have no periodicity,either. In addition,the numerical simulation results show that the interactions between femtosecond dark solitons are the same as those of picosecond dark solitons. It is found that their interactions agree with the empirical expression of picosecond dark solitons for the separation of two solitons as a function of initial separation and traveling distance. And the smaller the initial separation between two dark solitons is, the larger the difference between numerical simulation results and formula calculation results is.