The NOx emission characteristics of C $H_4$-Air coflow jet flames were numerically and experimentally investigated. NOx was measured using a chemiluminescent detection and calculated by the parabolic -type equation solver with a detatiled NOx chemistry. The fuel flow rate( $Q_{F}$), the diameter of mixture nozzle and the equivalence rate(Ф) were varied to discuss the EINOx of each flames at the various combustion conditions. The NOx emission index(EINOx) was introduced to quantify the NOx emission from the parametrically varied flames. The results show that Prompt EINOx increases on a logarithmic profile with increasing Ф and keeps nearly constant for the variation of $Q_{F}$. Thermal EINOx reaches the maximum value at around Ф =1.5 and then slowly decrease for Ф >1.5. In addition, Thermal EINOx increases with increasing $Q_{F}$, but nearly indifferent to the variation of the mixture nozzle diameter. Total EINOx also shows a peak at around Ф =1.5, followed by a relatively sharp decrease for 1.5< Ф <2.5 and increase slowly for 2.5< Ф < $infty$ The present Total EINOx trend is well explained by a combination of above Thermal and Prompt EINOx trend with the variation of Фn of Ф