The main purpose of this study was to examine the effect of Peer Scaffolding on students' grammar proficiency development. The recently developed model under study--Peer scaffolding--is based on Vygotsky's and Leentevo's Scaffolding instruction. To put the model into practice an intervention program was designed and implemented. The program had two components Experimental and Control group. The sample consisted of 102 grade eleven students in two classes at Tillili Preparatory and General Secondary School. The classes were randomly assigned into experimental and control groups. A non equivalent group pre-test-post-test quasi-experimental design was employed with 52 experimental and 49 control participants. Two grammar proficiency tests (one pre test and one post test) were designed. Both experimental and control groups were involved in similar scaffolding grammar proficiency activities for two months. The experimental group learned grammar activities using the peer scaffolding instruction. The control group, on the other hand, learned grammar the same exercise within the same period of time working with the teacher who was not trained in peer scaffolding. At the post intervention phase, the other tests followed by the social validity questionnaires were administered. The findings revealed that the Experimental group showed greater pre- to post-intervention improvement in grammar proficiency and show the intervention as socially valid. Thus, the study corroborated the efficacy of Peer Scaffolding intervention as feasible tool to enhance grammar proficiency development.