Positive solutions were studied for the nonlinear thirdorder periodic boundary value problem u(t)+ρ3u(t)=f(t,u(t)), 0<t<2π, u(i)(0)=u(i)(2π), i=0,1,2, where the nonlinear term f(t,u) is allowed to be singular at t=0,t=2π and u=0. By considering the integrations of height functions of nonlinear term on some bounded sets and applying the GuoKrasnosel′skii fixed point theorem, the existence of n positive solutions was proved.