Abstract: An incubation experiment was conducted to study the effect of two different mixtures of organic acids on availability of P in a red soil (RS) from Jiangxi and a black soil (BS) from Jilin. One mixture was low-molecular-weight organic acids (LA), the other was low-molecular-weight organic acids which was mixed with selected amino acids (LAA) under different levels. The results showed that different levels of LA and LAA improved Olsen-P level significantly in Jiangxi red soil and Jilin black soils compared to their corresponding control treatment (CKp).Treatments of half rate of LA (0.5 LA) and LAA (0.5 LAA) were more effective than treatments with normal rate of LA and LAA in Jiangxi red soil. Treatments with normal rate of LAA were more effective than treatments with normal rate of LA in red soil, while the treatments with normal rate of LAA was the most effective in Jilin black soil. On average, compared to the treatment of CKp，LA, 0.5LA, LAA, and 0.5LAA decreased P fixation rates by 10.5%,22.3%,11.3% and 19.4%, respectively in Jiangxi red soil, and by 8.6%,10.6%,14.1% and 11.8%, respectively in Jilin black soil. In a word, LA and LAA could reduce P fixation rates significantly in red soil and Jilin black soil. Treatments with half rate of LA or LAA (0.5LA and 0.5LAA) were more effective than that with normal rate in the red soil, while normal rate treatment of LAA showed better than other treatments in Jilin black soil.