The current research aims to investigate the effect of incidental learning strategy on EFL university students vocabulary acquisition and retention. In the light of these aims, the researcher has set two null hypotheses to be tested. The researcher adopted the one-group experimental design for this research. Accordingly, a sample of 40 students has been intentionally chosen from among second year students at the Dept. of English/ College of Basic Education/ University of Mosul for the academic year 2019/ 2020. The instrument has been prepared by the researcher to collect the data based on the dependent variables of this research; namely vocabulary acquisition and retention. An achievement test in vocabulary acquisition and retention was constructed by the researcher in the light of the behavioral objectives of the instructional materials. The validity and reliability of the instrument was verified and tested. The collected data were statistically analyzed by using different statistical means. The findings of this research show that the incidental learning is an effective strategy in acquiring vocabulary.