In this paper, the phase detection properties of a grating-coupled surface plasmon resonance (GCSPR) sensor with a thin metal film on the grating structure has been studied by performing finite-difference time-domain simulation first. Both the metal film thickness and modulation height of the grating considerably affect the phase detection properties of GCSPR sensors. The manner in which the metal film thickness affects the phase curve in the grating-coupled configuration is slightly different from that in the conventional prism-coupled surface plasmon resonance (PCSPR) configuration. For experiment, an electro-optic heterodyne interferometer is used to perform phase detection of the GCSPR sensor and a refractive index resolution of 1.5 × 0(-6) RIU are obtained. The results reveal that the phase detection sensitivity of the GCSPR sensor may be comparable to that of the PCSPR sensor.