One of the most influential models of working memory (WM) is the one developed by Baddeley (1986, 2000, 2003) which views WM comprising several components--a central executive, an episodic buffer, the visuo-spatial sketchpad, and the phonological loop. The phonological loop or phonological memory (PM) deals with the temporary storage of verbal and speech-based material. This study aimed to examine the effect of dual n-back task training on the expansion of PM capacity (PMC). One hundred twenty six adult EFL learners at the beginner level were randomly assigned to the control and experimental groups. The participants in the experimental groups were involved in dual n-back task training for six weeks. Nonword repetition and nonword recognition tests were used to measure PMC. The results showed that the training on the dual n-back task had a positive and significant effect on the expansion of the participants' PMC.