Dedicated to Professor Katsuhiro Shiohama on the occasion of his 70th birthday.Let M be an n-dimensional complete locally conformally flat Riemannian manifold with constant scalar curvature R and n ≥ 3. We first prove that if R = 0 and the L n/2 norm of the Ricci curvature tensor of M is pinched in [0, C 1 (n)), then M is isometric to a complete flat Riemannian manifold, which improves Pigola, Rigoli, and Setti's pinching theorem. Next, we prove that if n ≥ 6, R = 0, and the L n/2 norm of the trace-free Ricci curvature tensor of M is pinched in [0, C 2 (n)), then M is isometric to a space form. Finally, we prove an L n trace-free Ricci curvature pinching theorem for complete locally conformally flat Riemannian manifolds with constant nonzero scalar curvature. Here C 1 (n) and C 2 (n) are explicit positive constants depending only on n.