This paper presents a method for coding pi-calculus in the COQ proof assistant- , in order to use this environment to formalize properties of the pi-calculus. This method consists in making a syntactic discrimination between free names (then called parameters) and bound names (then called variables) of the processes, so that implicit renamings of bound names are avoided in the substitution operation. This technique has been used by J.McKinna and R.Pollack in an extensive study of PTS [5]. We use this coding here to prove subject reduction property for a type system of a monadic pi-calculus.