Metacognitive intervention of listening has prevailed in L2 (second language) listening research in the past decade. However, rare research has linked metacognitive intervention with online listening. This study examines L2 learners' development of metacognitive awareness of listening through online metacognitive listening practice. A set of online metacognitive listening exercises were constructed, based on a metacognitive cycle that regularly guides learners through metacognitive processes of listening. Thirty-nine low-proficiency Chinese university EFL listeners from one intact class participated in the study and did online listening practice as individual outside-class homework for 14 weeks. The development of metacognitive awareness was measured by MALQ and enriched by the learners' reflective notes. Results reveal an inverted U-shape pattern in the development of metacognitive awareness and that the factors of metacognitive awareness develop asynchronously. Some factors appear more susceptible to listening task difficulty and more unstable in the development process.