Cotton (Gossypium hirsutum) and sesame (Sesamum indicum) are two major crops in Islamic Republic of IRAN. In the northeast of IRAN, most farmers' landholdings are less than 5 hectares; therefore cotton production cost is high. Agricultural experts suggest that one method for improving productivity is intercropping system. This research was done to study advantages or disadvantages of intercropping of cotton and sesame through investigation of Land Equivalent Ratio (LER) and competition indices.  The experiment was conducted during 2015 growing seasons in Cotton Research Institute (C.R.I.). It was included a Randomized Complete Block Design (RCBD) with 3 replications. Treatments were intercropping patterns of cotton (c) and sesame (s) (1c:1c, 1c:2s, 2c:1s, 3c:1s, 1c:3s, sole cotton, sole sesame).  Substitution method of intercropping was done.  Land Equivalent Ratio (LER), Relative Crowding Coefficient (k), Aggressivity (A), Competitive Ratio (CR), Actual Yield Loss (AYL), and Relative Value Total (RVT) were evaluated.  Results revealed that, Land Equivalent Ratio (LER), Relative Crowding coefficient (K), Aggressivity (A), Competitive Ratio (CR) values of cotton generally were more than sesame. Planting cotton in intercropping with sesame had a higher monetary profitability too. In this study the best intercropping pattern for acquiring a highest Land Equivalent Ratio (LER) and monetary profitable was (3:1).