In this study, we evaluated NOx production rates of diesel combustions occurred in a constant volume chamber of a rapid compression machine in order to investigate relationship between flame behaviors and NOx emissions. A total gas sampling device was used to measure the NOx concentration in total gases existing in the chamber at a designated time. An EINOx (Emission Index of NOx) production rate was evaluated on the time history of NOx concentration. Temporal temperature distributions in the chamber were measured with a high speed 2-color thermometry. Gas oil (JIS #2) was used as the fuel. The EINOx production rate increases with increasing injection pressure through temperature rises in flames due to enhanced mixing of fuel vapor with ambient air. An increase in the ambient pressure causes overlaps between flames formed around the nozzle, which reduces the flame temperature. Nevertheless, high reactant concentrations resulting from the elevated ambient pressure slightly increase the net EINOx production rate. A reduction of number of injection holes decreases the total surface area of flames formed in the chamber, and then decreasing the EINOx production rate. In this study, the EINOx production rate was modeled in terms of the ambient air density, flame volume and reaction rate coefficient of an elementary reaction in the Zel'dovich mechanism. There is a correlation between the results obtained from the prediction model and the EINOx production rate even though a large derivation remains in the result obtained under the high ambient pressure condition.