Monoculture oil palm plantations have serious negative impacts on tropical biodiversity. In order to mitigate the impact, alley-cropping system or polyculture practice might be helpful because it can provide habitat heterogeneity and milder microclimate. Arthropods are an important group for their ecosystem services but vulnerable to expansion of the plantations. To understand how alley-cropping systems affect richness of arthropods, we assessed terrestrial arthropods using 210 pitfall traps in five alley-cropping plantations (7-10 year old oil palms with bactris, bamboo, cacao, pineapple and black pepper) and two monoculture plantations (7 and 15-year old oil palm) in Peninsular Malaysia. We also measured vegetation and microclimate variables in each plantation. In total 50,155 individuals and 28 families of arthropods were collected. Hymenoptera, Coleoptera and Orthoptera were the most dominant orders. The abundance of all arthropods, that of predators and decomposers and family richness were 1.3-1.5 times higher in alley-cropping than in monoculture plantations. Among environmental variables, ground cover and soil moisture had a positive effect on abundance of arthropods and family richness. Hence, alley-cropping can support arthropod population better than monoculture system with tolerable microclimate and decent vegetation structure.