In Taiwan, many studies have paid attention to searching the relation between EFL learners' reading comprehension and vocabulary acquisition. However, few researchers have conducted studies to investigate the ways students apply lexical inferencing strategies to know the new words in a reading context. This study aimed at exploring the lexical inferencing strategies used by EFL learners with high and low proficiency levels when they encountered unknown words in a reading context. The results derived from the introspective think-aloud protocols were analyzed and examined for observable lexical inferencing strategies. It was found that the two groups of students systematically used different lexical inferencing strategies in knowing the meanings of new words in a reading text. The results imply that it is necessary to teach lexical inferencing strategy to EFL learners to enhance their reading and lexical abilities.