This study scrutinized the relationship between utilizing language learning strategies, academic fields, and reading ability in reading comprehension test performance of Iranian postgraduate EAP students. The participants were 947 students, who answered a reading comprehension test and a learning strategy questionnaire successively in one session. The gathered data were subjected to a set of parametric statistical analyses, including descriptive statistics, one-way analysis of variance, Tukey HSD and Duncan tests. The findings manifested significant differences among the participants in different fields in employing overall, direct, and indirect strategies. A statistically positive relationship was found between the participants’ reading ability and use of overall, cognitive, compensation, metacognitive, and affective strategies. The findings reflected that the actual ability of language learners was significantly influenced by some nonlinguistic factors, and the observed scores did not represent their true ability. The findings can provide an empirical evidence for Bachman’s (1990) as well as Bachman and Palmer's (1996, 2010) conceptual frameworks of language use due to the impact of nonlinguistic factors on language ability of L2 learners in test-taking process. The findings can help language teachers improve instructional reading programs, decrease error of measurement, and narrow the gap between more successful and less successful learners in different fields of study.