In this thesis, we first explain the gravitational lensing and microlensing. Then, we study the circumsrtellar disks. In order to investigate the effect of disks on the microlensing light curves, we consider the protoplanetary disks as the most suitable candidate around the Galactic bulge source stars which are lensed. We aim to 1) study the effect of circumstellar disks on microlensing light curves and then 2) estimate the detectability of such disks through photometry observation of microlensing events via WFIRST satellite. It will start working in near future and will observer the Galactic bulge in 6 72-day seasons with the cadence around 30 minutes. In the chapter (5) we explained how to simulate the proto-planetary disks in microlensing events and then studied how microlensing light curve are perturbed due to the existence of disks. We finally did a simulation for a big ensemble of microelnsing events and found that for 16 percent of simulated events the photometry signals of disks is higher than 5\\% which is detectable though nowadays telescopes.