Many empirical findings of previous studies have suggested a connection between motivational factors and L2 writing. Nonetheless, the impact of motivational factors on the genre-based L2 writing has not gained much attention. The present study explored the extent to which the characteristics of two writing tasks involving different genres interact with a learner's motivational disposition (regulatory focus) and, further, affect language production of writing. 106 essays collected from 53 university EFL learners were assessed in terms of linguistic complexity and accuracy. 2X2 ANOVA results revealed significant main effects of genre on lexical complexity (lexical variation and MSTTR). More importantly, there were significant interaction effects between genre and regulatory focus on both measures of the syntactic complexity (mean length of T-unit and clause per T-unit) and lexical complexity (lexical density). The findings highlighted the significance of learners' motivational dispositions in genre writing. Drawing on the findings, some pedagogical suggestions to ensure the effectiveness of writing tasks on L2 development were proposed.