Change in land use causes changes in soil properties and soil fertility, with long-term effects on ecosystem and crop productivity. This study determined soil fertility along sequential conversion of cropland to grassland in China’s Loess Plateau. Soil samples were collected in 2015 at two sites in the semiarid region, following the conversion of cropland to grassland. Soil particle-size distribution, bulk density, pH, organic carbon (OC), total nitrogen (TN), total phosphorus (TP), available potassium, and available phosphorus were measured in this study. In addition, we analysed the changes of soil OC, TN, and TP, and evaluated soil fertility after the conversion from cropland to grassland. The establishment of grassland significantly increased soil OC, N, and P content, especially in the 0–10 cm soil layer. The highest change in soil OC, N, and P content occurred 6–10 yr after land conversion. The measured soil variables did not change significantly after 10 yr of land conversion. The overall increase in soil fertility after the land conversion was 13% at one site and 26% at the other site. The results suggested that establishing grassland could enhance soil fertility in the semiarid Loess Plateau region of China, and this enhancement is optimal 6–10 yr after the establishment of grassland.