The nature of peer feedback and its impacts on writing in English has attracted much attention of researchers and educators. Recent studies have indicated various types of peer feedback and its positive effects on writing development. This paper presents the results of an investigation into the nature of peer feedback and its effects on learners' writing argumentative essays in a Vietnamese context. The study aimed to explore the types of feedback which competent and less competent learners employed when they reviewed their peers' argumentative paragraphs. The study also aimed to measure the effects of one's giving feedback on their ability to write argumentative paragraphs. The study followed a two-group experimental research design with the participation of twenty-four English learners at preintermediate level of English. Four instruments were used in the study: the worksheet for peer feedback to elicit learners' comments to peers' argumentative paragraphs, the writing tests to examine learners' ability to write argumentative paragraphs before and after the treatment, the feedback coding scheme to code learners' comments and the assessment scale to evaluate learners' argumentative paragraphs. The results showed that both competent and less competent learners generated different types of peer feedback when commenting on their peers' argumentative paragraphs. In addition, the extent to which competent and less competent learners used the types of feedback was the same. Remarkably, giving feedback enhanced the mechanics component of learners' argumentative paragraphs.