  We consider the sign problem for classical spin models at complex $\beta
=1/g_0^2$ on $L\times L$ lattices. We show that the tensor renormalization
group method allows reliable calculations for larger Im$\beta$ than the
reweighting Monte Carlo method. For the Ising model with complex $\beta$ we
compare our results with the exact Onsager-Kaufman solution at finite volume.
The Fisher zeros can be determined precisely with the TRG method. We check the
convergence of the TRG method for the O(2) model on $L\times L$ lattices when
the number of states $D_s$ increases. We show that the finite size scaling of
the calculated Fisher zeros agrees very well with the Kosterlitz-Thouless
transition assumption and predict the locations for larger volume. The location
of these zeros agree with Monte Carlo reweighting calculation for small volume.
The application of the method for the O(2) model with a chemical potential is
briefly discussed.
