本文旨在探討英文字彙數量對英語學習者之重要性。研究對象為82位崑山科技大學進修部一年級及三年級之四技學生。研究工具為2,000字等級之字彙等級測驗。研究結果顯示進修部學生英文常用字彙嚴重不足，大部分學生之常用字彙在1,000字以下。最後本研究就在職學生之特色背景提供適切之字彙教學建議，以提高進修部學生有效運用英語之基本能力。This study aims to explore the instrumental role of vocabulary in reading and writing as well as the significance of vocabulary size to learners of English as a second language. The 2,000-word level of the vocabulary levels test was adopted to measure the vocabulary size of students attending the 4-year program at the continuing education division of Kun Shan University of Technology. Eighty-two students coming from a first-year and a third-year class participated in the test. The findings indicate that the participants are far below the 2,000 high-frequency word level, which is commonly cited as the threshold vocabulary size necessary for basic language use. The study infers that a majority of the continuing education students’ vocabulary size is less than 1,000 words. Teaching programs appropriate to working students such as explicit vocabulary instruction, narrow reading and vocabulary learning strategies are suggested to expand students’ vocabulary size.