Abstract: Soil enzymes are the biochemical catalysts of soil nutrient cycle. Research efforts on enzymological mechanisms of soil nutrient cycle are vital in evaluating the state of soil nutrients in the piedmont region of North China Plain (NCP). The objectives of the study were to investigate: i) the distributions of soil enzyme activity in the soil profile and ii) the correlations between soil enzyme activity and nutrient content of different particle-size fractions. The soils were collected from long-term field experimental plots at the Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences. The distributions of soil urease and alkaline phosphatase activities in the 0~10 cm, 10~20 cm, 20~30 cm and 30~40 cm soil layers were analyzed. Then the collected soil samples were grouped into coarse sand (2 000~200 μm), fine sand (200~63 μm), silt (63~2 μm) and clay (2~0.1 μm) fractions using low-energy sonication. Next the correlations among soil enzyme activities and soil nutrients contents were determined for the coarse sand, fine sand, silt and clay fractions. The results showed that the activities of soil urease and alkaline phosphatase decreased with increasing soil depth and increased with decreasing particle size. Enzyme activities were significantly correlated with soil nutrient content in coarse and fine sand fractions, but not so much correlated in the silt and clay fractions. These results suggested that soil enzyme activities were important driving factors of soil nutrient cycle. The enzymes in coarse and fine sand fractions promoted in situ catalytic activities and contributed greatly to soil nutrient cycle. On the contrary, enzymes in the silt and clay fractions inhibited in situ catalytic activities via soil mineral absorption, limiting their contribution rate to soil nutrient cycle.