Abstract: The transition in spatially evolving pipe flow at Re=3000 is studied by direct numerical simulation of Navier-Stokes equation. By the disturbance imposed from the pipe wall in the form of periodic suction and blowing(PSB), slug structures that travel downstream are generated. Meanwhile, steady streamwise streaky structures are formed. After the slug structure moves out of the computational domain, with the development of the disturbance, the streamwise streaky structures lose their stability and break down, and the transition occurs again. This is a new kind of transition, and is referred to as "econdary transition".