Dynamic assessment originates in the Zone of Proximal Development (ZPD). Practicing dynamic assessment necessarily requires the development of ZPD. This study aimed to investigate the effect of peer-assisted mediation vs. tutor-intervention within dynamic assessment framework on writing development and the attitude of Iranian intermediate EFL learners. To do so, a quasi-experimental design and a questionnaire survey were carried out. After conducting a pilot study, a language proficiency test was administered to homogenize the two intact groups with 30 students in each. The writing part of the proficiency test was also considered as pre test. Then, the two groups were asked to fill out the pre-test attitude questionnaire. While in peer-assisted mediation group, the writing assignments were assessed by peers followed by feedback provided by them, in tutor-intervention group, the tutor assessed and provided feedback on the assignments during the instruction. At the end both the post test of writing and the questionnaire were administered. Comparing the post tests indicated that the peer-assisted mediation group outperformed tutor-intervention group. Although, peer-assisted mediation group had significant effect on the writing of the learners, for tutor-intervention, this effect was not significant comparing the pre and the post tests within groups. The study also revealed that both peer-assisted mediation and tutor-intervention significantly changed the attitude of the learners towards writing development between the pre and the post test of questionnaires, though the difference between their effects on the learners' attitude was not significant comparing the post tests of questionnaires.