This study investigated the effects of web-assisted instruction (WAI), as compared with sole use of traditional face-to-face classroom instruction, on student learning of English as a foreign language by adopting a pretest-posttest control-group quasi-experimental design. The participants included 183 students from four intact classes in a technological university in southern Taiwan. A standardized English proficiency test was utilized as the pretest and posttest to measure student progress over an 18-week period. The results showed that (1) WAI is more effective than traditional instruction alone in improving students' posttest performance, (2) active participants in the online course achieve higher scores than passive ones in the posttest, (3) WAI has no differential effects for students of different ability levels, and (4) the experimental students acknowledged the advantages of WAI, but held reserved attitudes towards WAI. This study suggests that web-based learning be incorporated into the curriculum to provide students with additional learning in English, that students be guided and monitored throughout the process to ensure online learning success, and that teaching strategies be developed to encourage students to take control and responsibility for their own learning.