Abstract: MCA-PPO/PS composites were prepared by melt-compounding PPO/PS and melamine cyanurate (MCA) as a halogen-free flame retardant. The structure of the composites was characterized by XRD and SEM, and their combustion behavior and flowability were investigated. It is discovered that the microstructure of MCA in the composites is the same as that of pure MCA after the sample preparation and processing procedure. Thus the flame retardant effect of MCA remains unchanged. MCA is dispersed evenly in the composites, with no obvious agglomeration. In comparison with that of pure PS, the oxygen index (OI) of PPO/PS with mass ratio of 100:100 increases by 5.4%, the peak heat release rate (PHRR) decreases by 33.1% and the total smoke release (TSR) increases by almost 1.5 times. With rise of MCA loading, the flame retardancy of the MCA-PPO/PS composite gradually improves, while the total smoke release (TSR) decreases remarkably. Meanwhile, the melt viscosity of the composites decreases and the flowability increases with MCA loading. The addition of 5%, 25% and 45% mass fraction of MCA to the MCA-PPO/PS composite can decrease the TSR by 43.7%, 82.6%, and 91.6%, respectively. The flame retardant mechanism of PPO/PS is condensed phase charring mechanism. The flame-retarding mechanism of the MCA-PPO/PS composite changes from condensed phase charring mechanism to gas phase diluting and cooling effect of polymer matrix with MCA loading. The addition of MCA can enhance flame retardancy, suppress smoke release and improve melt flowability of the MCA-PPO/PS composite simultaneously.