Abstract: This paper puts forward a method in which the adaptabilities of a robot to various pipe diameters areevaluated via the pipe diameter adaptive cocfficient A(A_0.1). With this method, 4 types of mechanisms areevaluated. Using the theory of mechanism evolution, various mechanisms for robots moving inside pipesare deduced. Also, intrinsic relations between these types are clarified and relation between the positivepressure on the driving wheel and mechanism parameter is given. Experimental research is conducted un-der condition of slant branch pipe.