  We calculate for the first time the finite size corrections in the massive
Thirring model. This is done by numerically solving the equations of periodic
boundary conditions of the Bethe ansatz solution. It is found that the
corresponding central charge extracted from the $1/L$ term is around 0.4 for
the coupling constant of ${g_0}=-{\pi\over 4} $ and decreases down to zero when
${g_0}=-{\pi\over{3}}$. This is quite different from the predicted central
charge of the sine-Gordon model.
