The Purpose: Despite a large number of studies in online learning, limited studies have focused on language learners' metacognition, motivation, and self-efficacy beliefs in the online learning setting. This study attempts to fill this gap by evaluating how the three factors impact learners' English learning achievement in online learning environments. Design: Surveys were administered two times. Data analyses were based on longitudinal mediation analysis. The participants were 590 undergraduate students in China. Findings: The results showed a positive and significant correlation among the four variables. Overall, self-efficacy beliefs predicted English learning achievement. The findings support the joint mediating role of metacognition and motivation on the effects of self-efficacy beliefs on English learning achievement. Value: The findings support the need to strengthen learners' metacognition, motivation, and self-efficacy beliefs for online English learning.