This study aimed to explore how participating in an extensive reading programme contributed to young Korean EFL learners’ incidental vocabulary acquisition and reading comprehension development, with L2 motivation and working memory as mediating variables. Twenty–nine primary school students (the ER group) participated in an eight–week programme during their school vacation which involved reading two low–level graded readers a week. The three instruments administered before and after the programme were: a vocabulary knowledge test with 150 words selected from the graded readers; a general reading comprehension test for young learners; and a questionnaire on four different motivational orientations (Ideal L2 Self, Ought–to L2 Self, intrinsic reading and extrinsic reading). Reading span and nonword span tests were also used to measure the participants’ working memory. On the immediate post–test, the ER group’s vocabulary knowledge level exceeded that of the control group, and they retained even more words on the delayed post–test. On the reading post–test, there were no significant changes in the scores for either group. From these findings, it can be said that the ER programme substantially contributed to the EFL learners’ vocabulary learning, while it had little effect on their reading comprehension. A comparison between the language test results and the ER participants’ L2 motivations showed that their Ideal L2 Self and intrinsic motivation had a moderate correlation with the post–reading test scores, suggesting that a positive view of their future–self and their desire to learn English contributed to an improvement in reading comprehension. No correlations were found between their motivation and vocabulary acquisition. On the other hand, the participants’ nonword and reading span results showed moderate to high correlations with both the pre– and post–vocabulary tests while their correlations with the reading tests produced mixed results.