In this paper, we investigate the growth rate and strain relaxation of Si1-xGex layers grown on Si substrates by UHV-CVD. The Si1-xGex growth rate is found to exhibit two different behaviors as a function of growth temperature. Above the temperature corresponding to hydrogen desorption (T-H), the SiGe growth rate first decreases with x, and finally becomes almost independent of x at higher values of x. Below T-H, the SiGe growth rate first increases with x, then shows a maximum and finally becomes almost independent of x at higher values of x. Using in-situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy and photoluminescence, we have clearly identified two distinct mechanisms for strain relaxation as a function of Ge content : at x=0.15 strain is relaxed by nucleation of misfit dislocations while the film surface remains smooth throughout growth and relaxation process. The introduction of dislocations is found to occur before lattice relaxation. At x=0.22, strain is first relaxed via formation of coherent islands before reaching the equilibrium critical thickness for dislocation nucleation. We also show that in-situ RHEED is a powerful technique to probe the evolution of film morphology in an ultra high vacuum chemical vapour deposition system.