One dimensional multijunction solar cell simulator: Version-1 (MSCS-1D: V-1) is developed by Advanced Photovoltaic Lab, Physical Instrumentation Division, BCSIR Laboratories Dhaka. This work is financially supported by the Ministry of Science and Technology, the People’s Republic of Bangladesh under Special Allocation Project. Mr. Abu Kowsar is the principal investigator of this project, Dr. Syed Farid Uddin Farhad and Khaledun Nahar Babi are the co-investigator of this project. Mr. Syed Nazmus Shakib, University of Dhaka, contributed here as a M.S. Thesis student. The user friendly interface and the dialogue box of this simulation software were accomplished by the assitance of Mr. Md. Masum Billah, Mr. Sujoy Dey and Sazzatul Yeakin, SoftCare IT.