The study investigated variation in the depth of vocabulary knowledge of L2 learners at the same vocabulary breadth and the learner-related factors affecting it. Two hundred and thirty-one EFL learners at high and low levels of vocabulary breadth (according to Vocabulary Level Tests) participated in the study. The study tested Vocabulary depth (Word Associates Test and Derivative Word Form Test) and learner-related factors (LLAMA Aptitude Test, Attitude/Motivation Test Battery, Vocabulary Learning Strategies Survey, and Learning Style Inventory). Based on the findings using a paired t -test and Pearson correlations in SPSS and Structural Equation Modelling in AMOS, the study confirmed that (1) learners at the same level of vocabulary breadth (particularly low level) possessed variation in the depth of their vocabulary knowledge (such as meaning-based vs. form-based and receptive vs. productive knowledge) and (2) among the learner-related factors, learners’ language aptitude and motivation had a significant impact on vocabulary depth. Based on the results, the study made some pedagogical suggestions.