NTRU is a new public key encryption scheme based on hard problems in lattice. Due to the linear operations, both encryption and decryption of NTRU are very fast. However, decryption of this scheme may be failed even for recommended parameters. In this paper, NTRU decryption failures are analyzed. As a result, the theoretical bound of parameters is presented, which guaranteed NTRU decryption failure free. In order to correct NTRU decryption failures, we designed a compensating algorithm. To our knowledge, this algorithm is better than any other ones for NTRU decryption failure correction. An example is given at the end of this paper