The purpose of this study is to investigate learning effects of online peer-support for generating critical questions and counter arguments in small group online discussion. The online peer-support includes descriptions of what to do and generic and domain specific examples of questions. About 45 students were recruited from an online introductory class on turfgrass management offered by a land-grant university. A field experimental time series design with control-group was employed. Data were collected from five discussion sessions, ten open-ended essay exams, and three multiple-choice exams through a semester. The results indicated that the peer challenge guidance helped learners to generate more challenges (F=2.465, p<.05). However, it did not improve the quality of challenges. The increased quantity of challenges alone might be not sufficient to activate learners' reflection and critical thinking or to improve meaningful interactions. Consequently, it did not influence learning, performance in multiple-choice questions on memory and comprehension and open-ended essay questions. (Contains 23 references.) (Author/AEF)