Context-oriented programming(COP) is a programming method that enables us to define behaviors that depend on the program's execution context. We propose an implementation method of COP that does not need language extensions. Objective-C has a dynamic object system in which we can replace arbitorary methods. Harnessing such ability of the language, we construct a mechanism realizing COP. In this paper, we introduce how to construct such mechanism in Objective-C and evaluate the runtime overhead of our implementation.