This paper describe a technique to determine automatic generation of matching function in order to get a similarity level of document by using genetic algorithm (GA). an evaluation function for the fitness of each chromosome was selected based on jaccard, cosine and horng-yeh's formulation. these formulations are common measure association in information retrieval. to intialize a population, we need first to decide number of genes for each individual and the total number of chromosomes (popsize) in the initial population. GA is basically based on natural biological evolution. the parent solution (chromosome) with the higher level level of fitness has a bigger similarity percentage of documents, while those with lowel level of fitness have less similarity percentage of documents. the esult of this system that by similarity level of documents, the user can retrieve the most relevan document from a database.