Flow structures in a cross-flow fan at Re = 5400 based on the blade chord length and the tip velocity at the outer radius are investigated using large eddy simulation with an immersed boundary method. An eccentric vortex near a stabilizer, reverse flow between an impeller and the stabilizer, flow separation on blade suction surfaces, and a recirculation region at the corner of inflow region are observed from simulations. To analyze the effects of these flow structures on the fan performance, the variation of the torque coefficient of each blade in the azimuthal direction is examined. Consequently, the flow separation on the blade suction surfaces produces oscillatory variations of the torque coefficient along the azimuthal direction, causing degraded fan performance. Moreover, the recirculation region is shown to produce a considerable loss and the blades nearby suffer from high torque.