Ferromagnetic order in two-dimensional (2D) van der Waals crystals has been attracting much attention recently. Remarkably, room temperature metallic ferromagnetism is realized in 2D Fe3GeTe2. Here we design a monolayer (ML) Fe3GeTe2 spin-valve device by attaching two ends to ferromagnetic electrodes and applying a magnetic field to these ferromagnetic electrodes. We investigate the spin-involved transport characteristics of such a spin valve by using ab initio quantum transport simulation. A high magnetoresistance of ∼390% is obtained and significantly increased to 450-510% after the gates are introduced. The magnetoresistance of the ML Fe3GeTe2 spin valve is insensitive to the strain modulation. Our study provides a potential option for magnetic storage applications and will motivate further studies in spintronics based on this class of materials.