This study aims to investigate Taiwanese EFL senior high school students' vocabulary knowledge and their use of vocabulary learning strategies. The participants were 202 senior high school learners from two different schools in central Taiwan. The research instruments of this study include the Vocabulary Learning Strategy Questionnaire (VLSQ), the Vocabulary Levels Test (VLT), the Vocabulary Knowledge Scale (VKS), and Test of English for International Communication (TOEIC) reading comprehension. After the three achievement tests and questionnaire had been completed, 18 students from both high proficiency and low proficiency groups volunteered to take part in the semi-structured interviews. The results of respondents' Vocabulary Levels Test (VLT) showed that on average, the vocabulary size of Taiwanese third year senior high school learners was approximately 2,000 words, which did not meet the level required for reading basic English authentic texts. Learners' breadth and depth of vocabulary knowledge and reading comprehension were found to be strongly intercorrelated. With reference to the use of vocabulary learning strategies, the participants of the current study were merely moderate users of vocabulary learning strategies. Regarding the gender differences in the use of vocabulary learning strategies, an independent samples t-test indicated that female learners not only outperformed their male counterparts in the overall strategy use, but.significant differences were aiso found in the use of determination strategies, social strategies. and metacognitive strategies. As for the strategy use between different proficiency levels, an independent samples t-test revealed that high proficiency students adopted overall vocabulary learning strategies more often than low proficiency ones. Also, significant differences were also found in determination strategies, social strategies, memory strategies, and metacognitive strategies between these two groups of learners. From the following semi-structured interviews, the results indicated that there were distinctive perceptions of learning vocabulary between high proficiency and low proficiency learners in the five major areas: (1) perceptions of knowing a word, (2) awareness of the importance of vocabulary knowledge in learning English, (3) learners' difficulties in learning second language vocabulary, (4) learners' approaches to useful sources, and (5) self-regulation in learning vocabulary.