The affordances of MMORPGs provide authentic contexts for vocabulary acquisition. This study conducted incidental vocabulary learning on an MMORPG and investigated the effects of prior knowledge. The prior knowledge in the study was including English proficiency and gaming experience. To evaluate the learning effectiveness, 12 target words were appeared in task dialogues and flashcards on the MMORPG. An experiment was conducted for 52 fifth-grade students. The results show that the learners with medium gaming experience level had significant learning effectiveness for the vocabulary positioned in task dialogues. On the contrary, except high gaming experience learners, the learning effects of flashcard were more influenced by English academic level. In other words, the vocabulary in gaming requirement condition were more noticed by medium gaming experience learners, and the vocabulary in non-requirement condition were more perceived by higher English proficiency learners.