The present study is an attempt to analyze the vocabulary of Japanese English as a Foreign Language (EFL) learners’ collaborative writing performance, as compared to individual writing performance, especially focusing on tokens (i.e. the total number of words), types (i.e. the number of different words), lexical diversity, and lexical sophistication: in this study, the Guiraid index is used as a measure of lexical diversity and the Lexical Frequency Profile (LFP) as a measure of lexical sophistication. Prior to the study, 128 university students in Japan were administered a Productive Vocabulary Levels Test (PVLT). Study participants were selected based on their productive vocabulary as determined by this test. As a result, a total of 90 Japanese EFL learners were chosen as participants in the comparative analysis: 30 participants engaged in a picture description task individually, whereas 60 did the same task in pairs, leading to 30 paired texts in total. The results of this study revealed that the participants who engaged in pair work produced more varied and lower-frequency words in comparison with those who worked individually, although pair work did not result in a significant improvement in the number of word tokens. In sum, collaborative writing involving peer interaction can help EFL learners improve access to their vocabulary and function as a pedagogical tool facilitating language learning and teaching.