The freezing front in a tumor during percutaneous cryoablation therapy was traced both analytically and numerically exploiting a bioheat equation. It has been shown that there exists a limiting size of the tumor, which one single cryoprobe can freeze at the maximum. The freezing front moves radially outward from the cryoprobe and reaches the end, where the heat from the surrounding tissue to the frozen tissue balances with the heat being absorbed by the cryoprobe. An excellent agreement between the analytical and numerical results has been achieved for the time required to freeze the tumor using the cryoprobe of a single needle. An analytical expression for estimating the limiting radius has been derived to give useful information for cryotherapy treatment plans.