Narrow reading has not received much attention from researchers regarding its contributions on L2 learners' background knowledge, vocabulary knowledge, and writing styles. This study therefore investigated the effect of narrow reading on English as a foreign language (EFL) learners' vocabulary learning. Twelve graded readers were selected and categorized into four sets: same author (Tim Vicary), same genre (crime and mystery), same title ("The Railway Children") and random readers. Each set contains three graded readers: one Level 1, one Level 2, and one Level 3. Fifty-six students were divided into four subgroups and each group took turns reading each set of the graded readers. Twenty-five unknown target words were selected to be tested in each category. After students had read a set of three graded readers, their vocabulary knowledge was assessed on three dimensions: form-meaning recall, sources, and use. The data were analysed using linear mixed-effects models (LMM), with the participants as the random effect, and text organization, vocabulary dimensions, time order, and reading text sequence as fixed effect variables. The results show that more vocabulary words were acquired from texts by the same author or random texts than from texts of the same title; the L2 learners recalled 61% of the source and 50% of the meaning, and 41% of the target words were used correctly. Students scored the lowest at Time 1 and the highest at Time 4, indicating that learning rates increased as they read more. Pedagogical implications are discussed, and future research directions are suggested.