Aim: Litter decomposition is a vital process of carbon and nutrient cycling in terrestrial ecosystems. Despite rapid declines in plant diversity worldwide, the plant diversity effects on litter decomposition, along with the factors driving their directions and magnitudes, remain uncertain. Location: Globe. Time period: 1985-2018. Major taxa studied: Plants. Methods: By synthesizing 492 paired observations of leaf litter mixtures and monocultures from 110 studies, we conducted a global meta-analysis of the effects of litter mixtures on litter decomposition rates, which were calculated as k coefficients from m_t/m₀ =e^-kt, where m_t/m₀ was litter mass remaining proportion corresponding to time t. Results: Litter mixtures on average increased litter decomposition rates by 5.6% (95% confident intervals, 3.0%-8.1%), and the effects of litter mixtures increased with litter species richness, the functional diversity of chemical traits (leaf C, N, P contents and C:N ratio) and phylogenetic diversity consistently across terrestrial ecosystems. The decomposer abundance and function, including soil fauna abundance, microbial biomass, and extracellular enzyme activities were positively associated with litter mixture effects on decomposition rates. The structural equation models accounted for 48.6% of the global variation in litter decomposition rates and revealed that the positive effects of litter functional diversity on decomposer abundance and function led to increased litter decomposition rates, while litter phylogenetic diversity had a direct effect on litter decomposition rates. Main conclusions: The functional diversity of the chemical traits and phylogenetic diversity, both as indicators for complementarity effects, are important drivers for increasing litter mixture effects on decomposition. The positive litter diversity effects on decomposition rates are mechanistically linked with soil fauna abundance, microbial biomass, and extracellular enzyme activities. Our results suggest that plant diversity, especially functional and phylogenetic diversity, increases decomposer abundance and function, and thus plays a key role in the carbon and nutrient cycling across terrestrial ecosystems.