Soil organic carbon(SOC) and soil Olsen-P are key soil fertility indexes but information on their relationships is limited particularly under long-term fertilization. We investigated the relationships between SOC and the percentage of soil Olsen-P to total P(PSOPTP) under six different 15-yr(1990-2004) long-term fertilizations at two cropping systems in northern China. These fertilization treatments were(1) unfertilized control(control);(2) chemical nitrogen(N);(3) N plus chemical P(NP);(4) NP plus chemical potassium(NPK);(5) NPK plus animal manure(NPKM) and(6) high NPKM(hNPKM). Compared with their initial values in 1989 at both sites, during the 11th to 15th fertilization years annual mean SOC contents were significantly increased by 39.4-47.0% and 58.9-93.9% at Gongzhuling, Jilin Province, and Urumqi, Xinjiang, China, under the two NPKM fertilizations, respectively, while no significant changes under the no-P or chemical P fertilization. During the 11th to 15th fertilization years, annual mean PSOPTP was respectively increased by 2.6-4.2 and 5.8-14.1 times over the initial values under the two chemical P fertilizations and the two NPKM fertilizations, but was unchanged in their initial levels under the two no-P fertilizations at both sites. Over the 15-yr long-term fertilization SOC significantly positively correlated with PSOPTP(r2=0.55-0.79, P<0.01). We concluded that the combination of chemical P plus manure is an effective way to promote SOC accumulation and the percentage of soil Olsen-P to total P at the two mono-cropping system sites in northern China. Soil organic carbon (SOC) and soil Olsen-P are key soil fertility indexes but information on their relationships is limited particularly under long-term fertilization. We investigated the relationships between SOC and the percentage of soil Olsen-P to total P (PSOPTP) under six different 15-yr (1990-2004) long-term fertilizations at two cropping systems in northern China. These fertilization treatments were (1) unfertilized control (control); (2) chemical nitrogen Compared with their initial values ​​in 1989 at both sites, during the chemical (NP); (4) NP plus chemical potassium (NPK); (5) NPK plus animal manure (NPKM) and 11th to 15th fertilization years annual mean SOC contents were significantly increased by 39.4-47.0% and 58.9-93.9% at Gongzhuling, Jilin Province, and Urumqi, Xinjiang, China, under the two NPKM fertilizations, respectively, while no significant changes under the no -P or chemical P fertilization. During the 11th to 15th fertilization years, a nnual mean PSOPTP were increased by 2.6-4.2 and 5.8-14.1 times over the initial values ​​under the two chemical P fertilizations and the two NPKM fertilizations, but was unchanged in their initial levels under the two no-P fertilizations at both sites. Over the 15-yr long-term fertilization SOC significantly positively correlated with PSOPTP (r2 = 0.55-0.79, P <0.01). We concluded that the combination of chemical P plus manure is an effective way to promote SOC accumulation and the percentage of soil Olsen -P to total P at the two mono-cropping system sites in northern China.