Based on theories on vocabulary knowledge, grammar knowledge, and reading comprehension subcomponents, ten attributes/subskills were defined for 50 items from relevant subtests of TEM4 (Band Four of Test for English Majors in China). Cognitive diagnosis was conducted on the TEM4 data of the randomly sampled 2285 examinees (roughly at the B2 level) through the saturated generalized deterministic inputs, noisy "and" gate (G-DINA) model. The person parameters obtained from cognitive diagnosis served as the basis for simple multiple regression and path analyses for detecting relationship patterns. The study discovered that the relationship pattern at both construct and attribute/subskill levels can be better described as a mediation pattern in which vocabulary knowledge and its attributes are more suitable to serve as the starting point for reading comprehension. The study also discussed the patterns of the impact of vocabulary and grammar attributes on reading subskills as well as the internal subskill relationships within the construct of reading comprehension.