Abstract: In this paper the theory of plastic strain gradient has been put forward and thereby the problem is discussed on whether the deformational behaviour of the rock of class Ⅱ is exist. It has been proved that the existence of the deformational behaviour for the rock of class Ⅱ is conditional, that is , when 1-2 π EλlL>0 the behaviour manifests itself. The influence of the height of rock specimen, the descending modulus λ of rock and the internal length l on the class Ⅱ behavior has been discussed.