A method to generate test patterns referred to as defect aware test patterns is proposed. Defect aware test patterns have greater ability to detect un-modeled defects. The proposed method can be used with any test generation procedure to improve the effectiveness of the tests in detecting un-modeled defects. Experimental results on several industrial designs show the effectiveness of defect aware tests. We also propose a measure to estimate the effectiveness of given test sets in detecting un-modeled defects.