Research into metacognition has found it to facilitate self-regulation and correlate to learners' L2 writing level. Following Lee & Mak's (2018) framework of Metacognitive Instruction (MI) for L2 writing classrooms, this study applies Dynamic Assessment (DA) to writing MI (MI-DA) in a rural middle school EFL class in China. A one-semester comparative experimental study was conducted in two parallel Grade Seven classes (32 learners in each, taught by the same teacher) following a 3-step procedure: a nondynamic pretest and posttest for both control class (CC) and experimental class (EC) and an intervention phase, with CC receiving a score on written assignments and teacher's comments while EC was provided with MI-DA intervention during pre-writing, writing, and revision. Ratings of student independent writing as well as interview data indicate that MI improved significantly students' writing performance and metacognitive competence, influencing their attitude toward and confidence with writing. These goals, typically beyond the focus of most conventional assessments, are realized in DA through its commitment to taking account of the results of past development and those abilities that are ripening (i.e., future development).