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MoS 2 phototransistor has been widely investigated for its high sensitivity to light ranging from visible to near-infrared owing to the layer-dependent bandgap of MoS 2 . However, most of the devices in the previous studies employed a back-gate device structure, which limits its future practical application. Here, we take advantage of the high transparency of atomically thin graphene membrane to propose a top-gate phototransistor structure, in which the graphene acts as the top-gate electrode. Such MoS 2 photodetector exhibits ultrahigh responsivity reaching 1.4×105 AW−1 under the 550 nm incident light. The spectral response is also studied under the illumination of wavelength from 300 nm to 1000 nm. Other factors correlated with the lifetime of photogenerated carriers, including source-drain bias, gate bias, incident light intensity, are also systematically investigated.