We study the classical two-dimensional RP 2 and Heisenberg models, using the Tensor-Network Renormalization (TNR) method. The determination of the phase diagram of these models has been challenging and controversial, owing to the very large correlation lengths at low temperatures. The finite-size spectrum of the transfer matrix obtained by TNR is useful in identifying the conformal field theory describing a possible critical point. Our results indicate that the ultraviolet fixed point for the Heisenberg model and the ferromagnetic RP 2 model in the zero temperature limit corresponds to a conformal field theory with central charge c = 2, in agreement with two independent would-be Nambu-Goldstone modes. On the other hand, the ultraviolet fixed point in the zero temperature limit for the antiferromagnetic Lebwohl-Lasher model, which is a variant of the RP 2 model, seems to have a larger central charge. This is consistent with c = 4 expected from the effective SO(5) symmetry. At T > 0, the convergence of the spectrum is not good in both the Heisenberg and ferromagnetic RP 2 models. Moreover, there seems no appropriate candidate of conformal field theory matching the spectrum, which shows the effective central charge c ∼ 1.9. These suggest that both models have a single disordered phase at finite temperatures, although the ferromagnetic RP 2 model exhibits a strong crossover at the temperature where the dissociation of Z2 vortices has been reported.