This paper presents a study on all-atom molecular dynamics (MD) simulations of fibre���matrix interfaces in fibre-reinforced polymer (FRP) composites, with a focus on the effect of fibre sizing. The reactive force field ReaxFF was used in the simulations. The sizing treatment was found to significantly increase the interface toughness while has little effect on the peak stress of fibre���matrix interface. The characteristics of the fracture surfaces and the scission of C-O bonds observed in the simulations are consistent with pervious experimental observations. The developed modelling method opens up a new avenue of investigating the deterioration mechanism of FRP under combined mechanical-chemical-thermal actions.