The current study deals with a linguistic phenomenon in English entitled: Implied Negation on the Lexical Level which means the indirect use of negated words and phrases without employing explicit negators. It aims to (1) identify and classify errors made by EFL college students in using implied negation on the lexical level (2) find out whether there is any significant difference in students’ performance between the two areas of the diagnostic test. To achieve these aims, a diagnostic test has been constructed on forty students from the second stage at the College of Education for Humanities-University of Tikrit for the academic year 2024-2025. In order to ensure the face validity, the test is exposed to a jury of specialists in the fields of linguistics and methodology of teaching EFL who agreed upon its validity and suitability. Twenty students are randomly chosen for the pilot study which revealed that the time needed to complete the test items is 50 minutes. As for the clarity of the items, it is noticed that most of the items are clear and the students find no difficulty in answering the test items. As for reliability, the items are divided into two halves (odd and even). Using the Pearson correlation formula, it is found out that the test reliability is 0.79. After applying the Spearman Brown formula, it is found out the test reliability is 0.89. This shows that the test is quite reliable and acceptable. The results of this study show that EFL College students make errors of different sources of the test. It is also clear that EFL College students encounter more and complex errors at the second part of the test (Using vocabulary to express negative meaning) and that their performance is inefficient in this area. Finally, the study ends up with some conclusions.