Fine root respiration is a significant component of carbon cycling in forest ecosystems. Although fine roots differ functionally from coarse roots, these root types have been distinguished based on arbitrary diameter cut-offs (e.g., 2 or 5 mm). Fine root morphology is directly related to physiological function, but few attempts have been made to understand the relationships between morphology and respiration of fine roots. To examine relationships between respiration rates and morphological traits of fine roots (0.15–1.4 mm in diameter) of mature Quercus serrata Murr., we measured respiration of small fine root segments in the field with a portable closed static chamber system. We found a significant power relationship between mean root diameter and respiration rate. Respiration rates of roots < 0.4 mm in mean diameter were high and variable, ranging from 3.8 to 11.3 nmol CO₂ g⁻¹ s⁻¹, compared with those of larger diameter roots (0.4–1.4 mm), which ranged from 1.8 to 3.0 nmol CO₂ g⁻¹ s⁻¹. Fine root respiration rate was positively correlated with specific root length (SRL) as well as with root nitrogen (N) concentration. For roots < 0.4 mm in diameter, SRL had a wider range (11.3–80.4 m g⁻¹) and was more strongly correlated with respiration rate than diameter. Our results indicate that a more detailed classification of fine roots < 2.0 mm is needed to represent the heterogeneity of root respiration and to evaluate root biomass and root morphological traits.