Transition from conventional tillage to no-tillage may alter the depth distribution of soil organic carbon (SOC) and its chemical composition. The effects of 15 yr contrasting tillage systems on soil aggregation and the concentration and chemical composition of SOC were investigated in a semi-arid and semi-humid area of North China. Three treatments were established in 1996, including conventional tillage with (CTS) and without (CT) straw incorporation, and no-tillage with straw mulching (NTS). ¹³C nuclear magnetic resonance (NMR) spectroscopy was used to quantify the chemical characteristics of SOC in the 0–10 and 10–20 cm layers. NTS had a higher SOC stratification ratio than the CTS and CT. Aggregate stability, as indicated with the mean weight diameter, was higher for NTS and CTS than that for CT. SOC occluded in the >1 mm fraction was higher under NTS and CTS than under CT. Both NTS and CTS had more aliphatic carbon than CT did, and CT contained more aromatic carbon in the 0–20 cm layer. We concluded that NTS increased aggregate stability, enhanced SOC stratification, and altered the chemical composition of SOC in the plow layer, and could be a viable option for improving soil quality in the study area.