According to previous research, similarity of partners in a variety of characteristics is important for their mating. The similarity of the partners in certain traits also affects relationship satisfaction. The purpose of this study was to examine assortative mating in personality traits of the Big Five model, examine the mechanisms of assortative mating and effects of similarity in personality traits on relationship satisfaction. The study was conducted on 215 couples, aged between 19 and 65, who were at least one year in a relationship. In this study, participants completed the Big Five Inventory (BFI), and three measures of relationship satisfaction (Marriage Stability Scale, Partner's Satisfaction Index and Percieved Relationship Quality Components) which are merged into one standardized measure named Total Relationship Satisfaction. The results of this study confirmed assortative mating in extraversion and openness. Results also confirmed the initial assortment (partners are similar at the beginning of the relationship), as opposed to convergence. Partners actively seek for partners with similar traits (active assortment) and don't make choice by similar social background (social homogamy). Similarity in personality traits affects men's and women's relationship satisfaction. Men are more satisfied when both men and women in a couple have more pronounced and similar extraversion. Also, men are more satisfied with the relationship when they have a higher openness than women. Women are more satisfied with the relationship when men and women in a couple have more pronounced and similar openness.