  Through the Casimir effect, we obtain a temperature correction to the ground
state mass of the Bosonic string. Accordingly, we show that higher than the
temperature $\approx 1/2\pi $, the Tachyon attain real mass, and also we find
massive Graviton for nonzero temperatures. As another consequence, we
demonstrate that for temperatures higher than the Hagedorn temperature, the
string would find again a Tachyonic state.
