In this paper, the authors conducted a modal analysis of a drilling machine for blades of wind turbines. A simplified model of the machine was established with a commercial FEM software package, Ansys. At a specific working position the first six order natural frequencies were calculated and the results obtained from two different contacting manners were compared. The influence of static structural stress induced by gravitational load on natural frequency was also evaluated. The variations of the natural frequencies of the machine with the position angle of the rotary box were subsequently obtained. This is helpful for the determination of range of the excitation frequencies.