Organic montmorillonite (O-MMT) and ultra high molecular weight polyethylene (UHMWPE) were extruded and pelletized in the twin-screw extruder with the melt intercalation method to prepare exfoliated UHMWPE/O-MMT nanocomposites. The structural details of UHMWPE/O-MMT nanocomposites were characterized using XRD and TEM. In addition, the rheological properties of UHMWPE/O-MMT melts as a function of content of O-MMT were studied through dynamic and steady flow tests. The UHMWPE/O-MMT melts showed an enhancement of fluidity and a decrease of storage modulus compared with pure UHMWPE, which was the opposite of the rheological behavior of most polymer/layer silicate (PLS). A model based on confined phase was used to describe the structure and a disentanglement mechanism was proposed to explain the observed rheological properties of UHMWPE/O-MMT melts.