Mechanical creep behaviors of natural gas hydrates are of importance for understanding the mechanical instability of gas hydrate-bearing sediments on Earth. Limited by the experimental challenges, intrinsic creep mechanisms of nanocrystalline methane hydrates remain largely unknown yet at the molecular scale. Herein, using large-scale molecular dynamics simulations, mechanical creep behaviors of nanocrystalline methane hydrates are investigated. It is revealed that mechanical creep responses are greatly dictated by internal microstructures of crystalline grain size and external conditions of temperature and static stress. Interestingly, a long steady-state creep is observed in nanocrystalline methane hydrates, which can be described by a modified constitutive Bird-Dorn-Mukherjee model. Microstructural analysis shows that deformations of crystalline grains, grain boundary diffusion and grain boundary sliding collectively govern the mechanical creep behaviors of nanocrystalline methane hydrates. Furthermore, structural transformation also appears to be important in their mechanical creep behaviors. This study provides new insights into understanding the mechanical creep scenarios of gas hydrates.