Fine roots absorb water and nutrients and play a crucial role in ecosystem functions, possessing high adaptability to environmental changes. This study focuses on 14 common cultivated plants (eight woody plants and six herbaceous plants) in northeast China. After the addition of nitrogen (N) and phosphate (P) fertilizer, the specific root length (SRL), root diameter (RD), and root tissue density (RTD) of fine roots in different diameter classes (root diameter < 0.5; 0.5 ≤ root diameter ≤1; 1 ≤ root diameter ≤ 2) were measured. The effects of N and P addition on the morphological characteristics of the fine roots, and the relationships between these characteristics and their response to fertilization were explored. Results indicated that: N and P addition had a significant impact on the RD, SRL, and RTD of the roots in all the diameter classes. The SRL increased in most herbaceous species after N and P addition, and decreased in woody species at R<_0.5 (except Sa); N and P addition increased the RD of all diameter classes in most species but had inconsistent effects on the RTD of herbaceous and woody species in different diameter classes. A negative correlation was found between SRL and RD, regardless of fertilization or different diameter class, and the relationship between RTD and RD was not significant. The RD and RTD of woody plants were much larger than those of herbaceous plants, but the SRL of the former was lower than that of the latter. In summary, N and P addition had a considerable impact on root morphological characteristics of R<_0.5 and had a large effect on woody plants. This study provides insights into changes in the adaptive strategies of fine roots of woody and herbaceous species for N and P addition.