Steelmaking slag is a potential P resource in Japan, because the P quantity in it is almost equal to that in imported phosphate ores. P2O5 is mainly concentrated in the 2CaO・SiO2-3CaO・P2O5 solid solution in slag. It has been clarified that P can be selectively leached out from C2S-C3P rather than the matrix phase of slag. To recover P from slag, its dissolution ratio from C2S-C3P should be increased. In this study, the effects of leaching agent and Na2SiO3 modification on the dissolution of the C2S-C3P solid solution and slag in aqueous solutions have been investigated. H3C6H5O7 is beneficial for P dissolution from C2S-C3P in aqueous solutions because of the formation of the CaC6H5O7 - complex, which can suppress phosphate precipitation. The P-rich phase is changed from the original C2S-C3P to C2S-C2NP with higher water solubility by Na2SiO3 modification, which facilitates P dissolution. At pH=5, 85.7% of P from the modified C2S-C3P can be dissolved in the H3C6H5O7 solution. The selective leaching of P from the CaOSiO2- Fe2O3 system slag can be achieved by Na2SiO3 modification and leaching in the H3C6H5O7 solution at pH=5. The P dissolution ratio reaches 78.4%, and only 19.7% of Fe is dissolved.