Abstarct: Abstract The biggest challenge for researchers in the field of solar cells is the low efficiency. Several methods are used to increase efficiency. One of these methods is the junction with grading. In this work, CIGS solar cells were first introduced and then the difference between graded junction and abrupt junction was discussed. In the following, a review of the articles related to this field was done, and finally, the simulations that were done in this work using the grading were reviewed and analyzed, and their characteristics were recorded. This research was done theoretically with SCAPS-1D software. By comparing the models simulated in this work, the best type of grading for the buffer laxyer and also the best type of grading for the absorber laxyer are selected and the combination of these two gradings is used for the final model. In the final model where linear grading is used in it; these results were obtained. Open circuit voltage was 1.23 ev. short circuit current was 53.229 mA/cm. filling factor was 89.84% and cell efficiency was 58.88%. Therefore, the improvement of the performance of solar cells using grading was proved. It is worth mentioning that the overall efficiency in this thesis has been above 58%, while in the most of the theoretical researches in the field of CIGS solar cells, the efficiency has barely reached above 40%.