A one-dimensional (1D) imperfect annihilation reaction in the gas phase is studied via computer simulation. Particular attention is paid to the effects of the spatial distribution of particles on the reaction kinetics. Compared with classical kinetics, the imperfect reaction shows anomalous kinetics by the slowdown of the reaction rate and the anomaly is alleviated when the reaction probability upon contact is lower. The classical rate law is approached when the reaction probability approaches zero. The inspection of the spatial structure of particles shows that the slowdown of the reaction rate is caused by two major factors; the aggregation of the same velocity-species and the segregation of the different velocity-species. When the reaction probability is reduced, these two effects are weakened, and consequently the classical rate law is approached. Other features of the spatial distribution of particles and their effects on the kinetics are also discussed.