The dissociation of N2+2 in intense linearly polarized and circularly polarized femtosecond laser fields (45 fs, 5×1015-1×1016 W·cm-2) is studied at a laser wavelength of 800 nm and the experiment is based on time-of-flight (TOF) mass spectra of N+ fragment ions. By analyzing TOF mass spectra and the kinetic energy of N+ ions, we found that the dissociation mechanism of N2+2 in linearly polarized laser fields is different from that in circularly polarized laser fields. In linearly polarized laser fields, N2+2 ions are formed at the equilibrium separation RE of N2 molecules by sequential ionization and the kinetic energy release of the dissociation channel is consistent with predictions of the one-photon-absorption model. In circularly polarized laser fields, N2 molecules are ionized to N+2 ions first and then two atomic cores of N+2 ions start to separate from each other. When the internuclear separation reaches the critical separation RC (>RE), a dissociation occurs after the N+2 ion loses an electron. The kinetic energy release of the dissociation channel can be interpreted by Coulomb repulsion model.