Dependence of tearing mode saturation state on the current sheet thickness is investigated by two-dimensional (2-D) full particle simulations. When the system length Lx is taken to be the wavelength of the maximum growth mode Lx = λmax = 12D (D: half-thickness of the current sheet), the instability is found to saturate without producing significant reconnection if D > Dcr1st = 3.5 λe, where λe is the electron inertial length. When the system length is doubled Lx = 2λmax, only insignificant effects are available for D > Dcr2nd = 2.7λh, where λh is the ion-electron hybrid inertial length. Comparing these 2-D results with a recent 3-D result, it is shown clearly that a three-dimensional effect reduces the current sheet thickness and thus leads to quick production of substantial reconnection even if D > Dcr1st.