A novel salt form of (R)-3-[2-(2-hydroxymethylpyrrolidin-1-yl)ethyl]-5-methyl-2H-isoquinolin-1-one.The compound is (R)-3-[2-(2-hydroxymethylpyrrolidin-1-yl)ethyl]-5-methyl-2H-isoquinolin-1-onemonophosphate. Compared to the free form and monohydrochloride dihydrate,this monophosphate is chemically stabler, has higher solubility, and changesless in weight with changing moisture. It is an excellent compound for use asa raw drug for medicines.