Catalyst fragmentation behavior and polymer particle morphology evolution are two critical factors for obtaining high-quality polyethylene (PE). However, for metal-organic frameworks (MOFs), emerging catalysts for ethylene polymerization, systematic studies related to their fragmentation behavior and PE morphology evolution from early stages are still missing. Here, we provide a strategy by using MOF thin films as 2D model catalysts to elucidate PE growth and fragmentation behavior of MOFs in ethylene polymerization. The morphological evolution and particle expansion processes of the formed PE are observed as a function of polymerization time, which is linked to the morphology of MOFs and the crystallinity of produced PE. Moreover, the fragmentation process is visualized, revealing a combination of layer-by-layer and bisectioning fragmentation modes at very early stages and a dominant bisectioning fragmentation later. The desirable fragmentation behavior of MIL-101 (Cr) and PE growth and morphology indicate the potential of MOFs in ethylene polymerization.