The problem. This research study explores an important issue in the field of TESOL (Teaching English to Speakers of Other Languages) and second language acquisition (SLA). Its purpose is to examine the relationship between Vietnamese students' L1 grammar knowledge and their English grammar proficiency. Furthermore, it investigates the extent to which English grammar proficiency and language transfer in Vietnamese speakers of English are influenced by linguistic factors (first and second language grammar proficiency levels) and non-linguistic factors (the effects of age at the onset of learning English and the length of U.S. residence). Method. This study examined the relationship between Vietnamese students' L1 grammar knowledge and their English grammar proficiency and their language transfer. These issues were further analyzed in relation to age at the onset of learning English, length of U.S. residence, and L1 and L2 grammar proficiency levels. Ninety one participants, age range of 18-65, were the Vietnamese-American students from universities and colleges in Southern California. Results. Through the questionnaires of demographic and educational backgrounds and the results of the test scores, participants' Vietnamese and English grammar proficiency and language transfer levels were reported. Their performance in three tests, Vietnamese and English grammar tests, Vietnamese-English translation test, and the questionnaire were analyzed to investigate whether there was the correlation between Vietnamese L1 grammar and English L2 grammar proficiency. Additionally, these results also examined English grammar learning and language transfer in relation to age at the onset of learning English, length of U.S. residence, and L1 and L2 grammar proficiency levels. Regarding the relationship between Vietnamese L1 and English L2 grammar proficiency, the study showed that there was no significant correlation between the Vietnamese students' L1 grammar proficiency test scores and their L2 English grammar proficiency test scores. Furthermore, L2 grammar proficiency and language transfer varied depending on age at the onset of learning English, L1 and L2 proficiency levels, and areas of language and linguistics. In the area of the effect of the length of U.S. residence on L2 grammar proficiency and language transfer, the study indicated the influence of length of U.S. residence only on language transfer and the variance depended on areas of language and linguistics. The quality of L2 input, rather than length of U.S. residence, may affect L2 grammar learning and language transfer as well, which may be an inspiration for future research. Other suggestions for future research are further provided. The results of this research study have several pedagogical implications: The study has an implication for retaining native language(s) as learning several languages simultaneously does not cause serious impediments, but rather fosters language interdependence. Additionally, further pedagogical implications of this study are of great importance because the results can be applied not only to Vietnamese L1 speakers, but also to ESL/EFL learners from various L1 backgrounds. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]