Present day malware shows stealthy and dynamic capability and avails administrative rights to control the victim computers. Malware writers depend on evasion techniques like code obfuscation, packing, compression, encryption or polymorphism to avoid detection by Anti-Virus (AV) scanners as AV primarily use syntactic signature to detect a known malware. Our approach is based on semantic aspect of PE exectable that analyses API Call-grams to detect unknown malicious code. As in--exact source code is analysed, the machine is not infected by the executable. Moreover, static analysis covers all the paths of code which is not possible with dynamic behavioural methods as latter does not gurantee the execution of sample being analysed. Modern malicious samples also detect controlled virtual and emulated environments and stop the functioning. Semantic invariant approach is important as signature of known samples are changed by code obfuscation tools. Static analysis is performed by generating an API Call graph from control flow of an executable, then mining the Call graph as API Call-gram to detect malicious files.