The NOx emission characteristics of jet flames fueled with $H_2;and;CH_4$ were studied. Experimental and numerical investigations were carried out for various flames with varying equivalence ratio, fuel flow rate and nozzle diameter. The Emission indices of NOx(EINOx) were measured by chemiluminescent method and calculated by numerical model based on detailed chemistry. The results show that EINOx of $CH_4;and;H_2$ flames have different trends in terms of equivalence ratio and fuel flow rate but have the same trends in terms of nozzle diameter. These differences can be explained by the following Thermal and Prompt trends in both flames. Thermal EINOx is quite sensitive to the residence time in the high-temperature region weighted by the maximum flame temperature. Prompt EINOx is mainly influenced by flame surface area of each combustion conditions.