Fusion between viral and cellular membranes is the essential first step in infection of enveloped viruses. This step is mediated by viral envelope glycoproteins (Env) that recognize cellular receptors. The membrane fusion between the effector cells expressing viral Env and the target cells expressing its receptors can be monitored by several methods. We have recently developed a pair of chimeric reporter protein composed of split Renilla luciferase (RL) and split GFP. We named this reporter dual split protein (DSP), since it recovers both RL and GFP activities upon self reassociation. By using DSP, pore formation and content mixing between the effector and target cells can be monitored upon the recovery of RL and GFP activities after the membrane fusion. This quick assay provides quantitative as well as spatial information about membrane fusion mediated by viral Env.