The purpose of this study is to test whether phonological decoding process contributes to vocabulary learning in the frame of task involvement load. Three task materials with different amount of loads were created. Thirty eight Korean EFL university students participated in the task experiments (13 for reading comprehension task, 14 for reading comprehension plus decoding process, and 11 for reading comprehension plus gap-filling task). The participants’ performance was tested three times (pre-test, immediate post-test, and delayed post-test). It was found that there were no significant differences among the three groups in the immediate test, and that the reading plus phonological decoding group was significantly more advanced than the reading comprehension group and the reading plus gap-filling group in the delayed test. The results indicated that phonological decoding process was conducive to successful retention of vocabulary knowledge, a lot better than the reading comprehension task and the gap-filling task. It is suggested that phonological decoding needs to be regarded as one of the components of involvement loads that English instructors create various task materials entailing students’ phonological process.