Regarding to the microcavity structure of VCSELs the relationship between RIN and external optical feedback level with different aperture size,spontaneous emission factor and linewidth enhancement factor is analyzed.The results indicate that the sensitivity to external optical feedback of VCSELs can be reduced by using a smaller aperture size or linewidth enhancement factor.But the spontaneous factor has a negligible influence on the sensitivity.These characters can hardly hold true in edge-emitting lasers because of the limitations of their structure.