By establishing one control and two experimental groups consisting of 82 learners of English as a foreign language, this study explored the relationship among reading comprehension ability, grammar and vocabulary knowledge to find out whether an experimental design will confirm the findings of Shiotsu and Weir (2007) supporting the superior relationship of grammar to vocabulary knowledge. The administration of the tests showed that the two experimental groups performed significantly better than the control group not only on the seen but also on the unseen reading comprehension tests (RCTs). However, the correlation coefficients obtained between the syllabus-based vocabulary test and the seen RCT and the unseen RCT was noticeably higher than the ones obtained between the syllabus-based grammar test and the seen RCT and the unseen RCT. The implications of the findings are discussed and suggestions are made for future research.

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