The present study aimed to investigate the comparative impacts of input-based and output-based activities on vocabulary knowledge of Iranian EFL learners. To fulfil this objective, 54 intermediate EFL students out of 70 from a private language institute were chosen through administering the Oxford Quick Placement Test (OQPT). The selected participants were divided into one control group (n = 18) and two experimental groups including the input-based group (n = 18) and the output-based group (n = 18). Afterwards, all three groups took a productive vocabulary test as pre-test. Then, the experimental groups received the treatment. The experimental group 1 (input-based group) received the instruction through input-based activities and the experimental group 2 (output-based group) received the instruction through output-based activities. The whole treatment lasted for 9 sessions of 50 min. After the treatment, the post-test of productive vocabulary test was administered to the all groups. Also, after two weeks, a delayed post-test was administered to the learners to examine the effect of different input and output-based activities on EFL learners' vocabulary retention. The results of, one-way ANOVA, and Scheffe post hoc tests revealed that both input-based and output-based groups outperformed the control group on the post-test and delayed post-tests. However, there was not a significant difference between the performances of the experimental groups on the post-test and delayed post-tests. The findings provide further evidence that both input-based and output-based activities lead to both productive vocabulary knowledge. In general, the results show similar levels of effects for input-based and output-based activities on vocabulary acquisition. The implications and suggestions for future research are also presented.