The Mathematics and Natural Sciences Olympiad is a competition event that aims to hone as well as provide students in the fields of Mathematics and Natural Sciences. The selection of prospective participants to attend the year event at each school has their own suggestions and methods.  Based on the above the authors conducted a study of a computer-based system for the development of participant selection methods, using simple additive weighting methods and using 6 proposals such as mathematics tests, science tests, average report cards, discipline, activeness, and honesty in give weight. After conducting the testing and testing process on the support system decision on the selection of candidates for Mathematics and Natural Sciences Olympiad level elementary school students to use simple additive weighting method in SDN 2 Melayu, the result is a capable system provide recommendations for potential participants who are entitled to participate Olympic with 75% confidence level.