This study investigated the relationship between vocabulary learning strategies (VLS) used by English major undergraduates and their performance on vocabulary size tests (VLT and PVLT) in Taiwan. The aims of this research were fourfold : (a) to investigate the overall patterns bf VLS used by the participants, (b) to explore the correlation between the ten subscales of VLS and vocabulary size tests, (c) to look into the gender differences of VLS use as associated with their performance on vocabulary size tests, (d) to find out the incremental growth in vocabulary size tests among freshmen, sophomore, and junior participants and indentify which subscales were relevant to the four word levels of vocabulary size tests. The participants included three hundred and one English major students in the research location who successfully completed a modified VLS questionnaire along with combined vocabulary size tests for data collection. Data for this research were then analysed using statistical methods, descriptive statistics, correlation analyses independent sample t-tests, and linear regression analyses. The research findings shed light on the identification of the overall patterns of VLS use, the correlation between the VLS and vocabulary size tests, the gender differences in VLS use and performance on vocabulary size tests, and the identification of the progress of vocabulary acquisition and strategies relevant to specific levels of vocabulary size tests. The insights gained in this study may lead to raising the awareness of the policy makers, the school administrators,and the course designers and the teaching instructors in the research location who are interested in the relationship between vocabulary learning strategies used by these English major undergraduates and their performance on vocabulary size tests.