We consider in this letter the problem of blind frame synchronization of Low Density Parity-Check (LDPC) codes. We present and compare new methods of blind frame synchronization based on the calculation of Likelihood functions of the syndrome. Simulation results show that the four proposed synchronization techniques are effective once applied to LDPC codes. The method that involves the maximization of the Likelihood Difference (LD) function of the parity-checks, presents the best results in terms of probability of false synchronization.