In the S-P networks

the design of P-transformation is very important

and is related to integral diffusion performance of block cipher.Based on the importance of P transformation

we propose the concept of a class of special bit transformation.We prove that the special bit transformation is involutory transformation and it's branch number is 4.We also give the number of input-output correspondences whose weight sum equals to 4.By further analyzing

it is shown that the special bit transformation has good diffusion performance though it's branch number does not obtain maximum.