Previous studies have shown positive effects of self-assessment on L2 English learners' speaking performance, but it is unclear whether the same is true with peer assessment. Research is also scarce on the effects of such assessments on the learner's sense of autonomy. This study thus investigated the effects of self- and peer-assessment on L2 English speaking performance and learner autonomy. To take advantage of the information communication technologies in assessment, the online platform VoiceThread was used. While all participants received feedback from the same instructor, only the experimental group, known as the "online assessment group" used VoiceThread to narrate a self-assessment of their own uploaded speech videos and to narrate peer-assessments of their classmates' uploaded speech videos. Quantitative and qualitative data were collected from the 39 participants majoring in English at a 4-year college in Taiwan. Results showed that the experimental group outperformed the control group in both speech performance and sense of learner autonomy. In an open-ended survey, participants in the experimental group also expressed confidence that the online self- and peer-assessment helped them become better English speakers. These findings suggest that with proper guidance, L2 learners can become more proactive in improving their speech performance.