Abstract: Plant root decomposition is one of the critical processes of carbon and nutrient cycling in terrestrial ecosystems. Plant roots less than 2 mm in diameter constitute a heterogeneous branching system. Within the system, lower order roots (e.g. 1-3 order) or fine roots in smaller diameter (e.g. less than 0.5 mm in diameter) which mainly serve for water and nutrient uptake have fast turnover rates (0.5-2.5 times per year), and contribute greatly to soil carbon and nutrients pools. Recently, increasing number of studies have shown that lower order roots decompose more slowly than higher order roots (e.g. more than 3 order) or fine roots in larger diameter (e.g. more than 0.5 mm in diameter) which mainly serve for water-nutrient transport and carbon storage. Here, we review three main factors explaining the contrasting decomposition rates across different root orders or diameter classes: mycorrhizal, carbon quality and nitrogen content. Overall, we intend to provide insights into the important roles of functional traits (e.g. diameter) of fine roots that play in ecosystem carbon and nutrient cycling in a changing world.