This paper proposes a method for evaluating the artificiality of protected code by means of an N-gram model. The proposed artificiality metric helps us measure the stealth of the protected code, that is, the degree to which protected code can be distinguished from unprotected code. In a case study, we use the proposed method to evaluate the artificiality of programs that are transformed by well-known obfuscation techniques. The results show that static obfuscating transformations (e.g., control flow flattening) have little effect on artificiality. However, dynamic obfuscating transformations (e.g., code encryption), or a technique that inserts junk code fragments into the program, tend to increase the artificiality, which may have a significant impact on the stealth of the code.