Phosphorylation of nucleosides with sodium cyclo-triphosphate (P3m) was studied under various conditions (mixing ratio of P3m to nucleoside, pH, reaction temperature and time). 1) Adenosine, cytidine, guanosine, and uridine were easily phosphorylated by P3m to form selectively nucleoside 2′-monophosphates, 3′-monophosphates, and 2′,3′-cyclic monophosphates. On the other hand, 2′-deoxyadenosine, 3′-deoxyadenosine, 2′-deoxycytidine, and thymidine could not be phosphorylated by P3m. 2) The phosphorylation of adenosine, cytidine, guanosine, and uridine varied strongly, depending on the reaction conditions; mixing ratio, pH, reaction temperature and time. Under conditions of a high mixing ratio of P3m to nucleoside (5:1–10:1), high pH (12–14), high temperature (70–100 °C) for a short time (1–2d) or low temperature (room temperature) for a long time (70–150 d), nucleoside 2′- and 3′-monophosphate could be obtained in high yield (about 50–100%). 3) At the initial stage of the phosphorylation reactions of adenosine, cytidine, and guanosine with P3m, nucleoside 2′,3′-cyclic monophosphates were formed, though a small amount (about 1–3%), but in the course of the reaction for a long time, they were hydrolyzed to nucleoside 2′- and 3′-monophosphates. 4) 2′-deoxyadenosine, 3′-deoxyadenosine, 2′-deoxycytidine, and thymidine could not be phosphorylated by P3m. This means that the presence of hydroxyl groups at the 2′- and 3′-positions of the ribose was necessary for the phosphorylation of nucleosides with P3m. 5) Phosphorylation of nucleosides with short-chain phosphates (ortho-, pyro-, and triphosphate) could not be observed at all. 6) The mechanism of formation of nucleoside 2′-monophosphates, 3′-monophosphates, and 2′,3′-cyclic monophosphates in the phosphorylation of nucleosides with P3m was discussed.