Two-species system are the most convenient system to study context dependence of multi-species because such system can be formulated in a mathematical model by the smallest set of population parameters (intrinsic growth rate, strengths of intra- and interspecific density dependence of each species). To understand context dependence of population dynamics in two-species system, we propose a new framework in which standardized intrinsic growth rates of each species are regard as the environmental suitability for these species and, then assess how strengths of intra- and interspecific density dependence of these species change depending on the environmental suitability. By using 18yr data of pairwise sessile species in rocky intertidal shore along eastern Hokkaido, we revealed how strengths of intra- and interspecific density dependence of these species change depending on environmental suitability. Both the strengths of intra- and interspecific density dependence decreased with the increase of the environmental suitability of focal species, which indicated that strengths of intra- and interspecific density dependence are mainly related to its own environmental suitability for the two-species system. By combining this framework with modern coexistence theory and environmental gradient-based approach can provide a deep understanding for the coexistence mechanisms and context-dependency in species interaction in two-species system.