Strain relaxation and misfit dislocation density in Si0.8Ge0.2 epilayers grown in micron size windows by MBE are studied. Reduction of misfit dislocation density and increment of strain relaxation in Si0.8Ge0.2/Si heterostructure grown by MBE in the windows smaller than 20 x 20 mum(2) were clearly observed. Besides, experiments showed that dislocation density in SiGe films would be different due to the different stress in the mask materials while the window size and the composition x of the SixGe1-x films were the same. Qualitative explanations for these experimental results are discussed.