Electric conductance of DNA molecules was studied theoretically. We have adopted the tight binding small polaron model and used the Landauer and the Kubo formalisms to calculate the molecular conductance both in the quasicoherent and the incoherent regime. In the experiment of the poly(dA)-poly(dT) and the poly(dG)-poly(dC) DNA molecules, a unique current-voltage (I-V) curve, I proportional to sinh(bV), was observed. The distinct temperature dependences of the coefficient b observed in the two DNA molecules may indicate that, although the poly(dG)-poly(dC) molecule is in the incoherent regime, the poly(dA)-poly(dT) is in the quasicoherent regime. The latter is explained by the quasidiagonal zero energy process of the nondiagonal small polaron hopping process. Besides the temperature dependences of b, the length dependences of the electric current in the two regimes were derived, which may have important implications for the future experiment.