Motivation has played an essential role in learning, leading to various research areas across diverse disciplines. While the literature is replete with findings supporting the essentiality of motivation in an individual's academic success, demotivation (a.k.a. negative motivation) has been underexplored, even though it has been a widespread phenomenon observed in language learning, particularly in the field of English writing in EFL settings. To keep leaners motivated and engaged, innovative pedagogies have been proposed, among which flipped learning and writing instruction have received extra attention. Previous studies have shown the effectiveness of flipped learning and online writing practices on enhancing student motivation and learning outcomes. Nonetheless, research on how the aforementioned instructional practices could be integrated to facilitate English writing among EFL learners still remains scarce, let alone a detailed probe into demotivational factors that might exist in such an innovative instructional design. Therefore, this study examined demotivation factors in EFL writing and the effects of the online flipped writing instruction on EFL writing proficiency. Forty-eight sophomore English-majors in Taiwan were paired with American college peers. Multiple sources of data over a semester included pre- and post-tests on two genres (descriptive/argumentative), 'Demotivation Scale', reflective journals, and interviews. The results revealed that the online flipped writing instruction enhanced the students' writing proficiency (both globally and locally), and that four characteristics of the flipped instruction were observed not to cause demotivation in English writing, including self-interest, classroom atmosphere and teaching material, writing product, and teaching method and content. The participants held positive perceptions about the online flipped writing instruction and such learning experience enhanced cross-cultural observation. Based on these findings, pedagogical implications are provided.