For an in-line arrangement of two drag-type hydrokinetic rotors, flow patterns between the two rotors and the rotor performance were investigated. The flows were measured using time-resolved particle image velocimetry (TR-PIV) technique. Computational fluid dynamics (CFD) technique was used to quantify the rotor performance. Emphasis was placed upon the influence of the rotor angle and the inter-rotor distance. The results show that two large-scale vortices dominate the wake of the upstream rotor. Expanding, shrinking and shifting of the large-scale vortices significantly influence the rotor performance. Three distinct regions, velocity-rising, velocity declining and velocity-recovery regions, are defined. They serve as indicators of the impact of the downstream rotor on the upstream rotor. Instantaneous wake flow patterns between the two rotors alter with the orientation of the two rotors. The most adverse effect on the upstream rotor arises as the upstream rotor area that faces the incident flow reaches its maximum. At high upstream velocity, both the torque output and the power coefficient of the upstream rotor approach their counterparts associated with the single rotor. With increasing inter-rotor distance, the negative impact of the downstream rotor on the upstream rotor decays continuously. An inter-rotor distance larger than five times the rotor diameter is suggested.