Since L1 comprehension is known to affect achievement of L2 reading comprehension as well as L2 general ability, the already-acquired L1 is considered to have a strong influence on L2 learning. The pair-matched sampling method was utilized for selecting 80 pairs of native Chinese and Korean speakers learning Japanese, by matching scores for general linguistic ability, duration of study, gender and age. Using SEM (structural equation modeling) analysis, the present study depicted L2 causal relations of lexical and grammatical knowledge upon discourse comprehension. Results suggested that Chinese speakers rely heavily on their lexical knowledge for discourse comprehension whereas Korean speakers use a balanced approach of lexical and grammatical knowledge for discourse comprehension. Consequently, the contrast between the two L1 linguistic backgrounds showed a clear strategic difference toward L2 Japanese discourse comprehension.