The Compton Spectro-Imager (COSI), formerly known as Nuclear Compton Telescope is a stratospheric balloon borne telescope which aims to produce images and spectra of the gamma ray emissions of the astrophysical sources in the range of 0.2-5 MeV. The aim of this thesis was to calculate or determine the performance of COSI by detection efficiency depending on the energy and the direction of arrival of photons and to participate in the calibrations and use the data obtained from the calibrations for imaging.After getting familiarized with the operating principle of COSI and learning to use the simulation tools from the MEGAlib (Medium Energy Gamma-ray Astronomy library), simulations were performed to determine the detection efficiency for 511 keV and 1809 keV.After simulations, real or raw data from the 12 detectors of COSI was used to do the energy calibrations using the MEGAlib tool called Melinator and further analysis was done using the tool called Nuclearizer. In the end, an image was achieved for the 662 keV (Cs-137) line.