For the initial growth of Si on Ge, three-dimensional Si quantum dots grown on the Ge surface were observed. With increasing Si thickness, the Si growth changes from three-dimensional to two-dimensional growth mode and the dots disappear gradually. Finally, the surface is smooth with the roughness of 0.26 nm, similar to the original Ge substrate, when 15 nm Si is deposited. More Ge segregation on the wetting layer leads to more open sites to increase the subsequent Si growth rate on the wetting layer than on the Si dots. The in-plane x-ray diffraction by synchrotron radiation is used to observe the evolution of tensile strain in the Si layer grown on Ge (100) substrate.