This study intended to investigate the effect of spaced and massed distribution instruction on vocabulary learning. To fulfil this objective, 68 Iranian pre-intermediate EFL learners (14-16 years) participated in 16 sessions. The participants were randomly divided in to two experimental groups; spaced distribution group (n = 34) and massed distribution group (n = 34). The massed distribution group had one intensive session on learning the target vocabulary; the spaced distribution group had three sessions at irregular time intervals. Using a before and after design, students were retested after 8 weeks. To collect data, a vocabulary test was performed as the pretest and posttest. The results of the paired samples t test and One-way ANCOVA indicated that the spaced distribution group significantly outperformed the massed distribution group on the posttest (effect size .75). The results propose that EFL practitioners can synthesize spacing as a beneficial teaching technique into the curricula and educational materials to promote vocabulary learning.