Abstract: The detection of unknown malicious executables is beyond the capability of many existing detection approaches.Machine learning or data mining method can identify new or unknown malicious executables with some degree of success.Bayes or improved Bayes algorithm has the detection capability of unknown malicious excutables;however,it takes more time to study.A new improved algorithm is proposed in this paper.The new classifier based on strings achieve has high detection rates and can be expected to perform as well in real-world conditions.