Optical tweezers are one of the most efficient means of manipulating objects on the micrometer scale and biological cells. The sources of the existing optical tweezers are CW light or long-pulse-width light. The concept of femtosecond laser tweezers was proposed. Taking femtosecond pulses as the sampling of CW lasers, the theoretical model of the induced axial forces by femtosecond laser pulses exerted on micrometer-sized sphere was established. The major factors that affect the trapping were discussed and suitable beam waists and radii of the trapped particles were demonstrated. The numerical results show that micron-sized particles can be trapped with femtosecond pulses with suitable single pulse energy, beam waist, pulse wavelength and the relative refraction index of the particle to the medium. Considering the axial optical force, the gravity and the Brownian inertial force, the pulsed property of the axial gradient force and the stability condition of femtosecond laser tweezers were also demonstrated.