Observations from satellite remote sensing provide important tools for regional and global monitoring of terrestrial ecosystems. This article evaluated a recently developed vegetation index (VI) model that estimates gross primary production (GPP) in an open grassland using flux measurements and 250-m moderate-resolution imaging spectroradiometer (MODIS) images. Two VIs, the two-band enhanced vegetation index (EVI2) and the normalized difference vegetation index (NDVI), were tested as indicators of both fraction of absorbed photosynthetic active radiation () and light use efficiency (LUE). The standard MODIS GPP product was moderately correlated with flux-measured GPP as indicated by the coefficients of determination () of 0.56 and a root mean square error (RMSE) of . In comparison, results suggested that the VI model can provide improved estimates of GPP, with of 0.80, 0.82, and 0.81 for photosynthetically active radiation (PAR), , and , respectively. The underlying reason was that both EVI2 and NDVI were potential proxies of ( of 0.43 and 0.53, respectively) and LUE ( of 0.52 and 0.46, respectively). The primary analysis will be helpful for spatial improvements of future GPP models.