In between the billions of stars in the Milky Way Galaxy, there is ISM. Despite the extremely low densities and low temperatures, ISM provides a hotbed of the formation of molecules. Up to now, over 100 species have been found towards the ISM, especially towards the cold and hot dense cores. Findings of polyatomic molecules in the ISM may contribute to the answer of one of the ultimate questions: what is the origin of life on the Earth? This makes the astrochemistry research a hot topic. One good example of much interest is the isomers made of Hydrogen, Carbon, Nitrogen, and Oxygen. The most stable isomer, isocyanic acid (HNCO), is a well-known interstellar molecule and has been detected towards many interstellar sources. However, two other metastable isomers, cyanic acid(HOCN) and fulminic acid(HCNO), have been only observed towards some of the sources where HNCO is seen. Gas-grain time-dependant chemical models are applied to reveal the mechanism. As a result, CHNO isomers in ISM are found to be formed by a combination of both gas phase and grain surface chemical reactions. In general, the models are able to reproduce the detected abundance of CHNO isomers in different regions.