Development of new burner technology is common practice nowadays. Standardized test codes are available to conduct tests to determine operating characteristics and thermal performance of different parts of steam generating units. This paper describes the results of testing performed to evaluate thermal efficiency of a 74.5 kW fire-tube steam generator fired with a new premixed flame burner in comparison to a typical standard-retrofit diffusion flame burner system. Two premixed flame burners sized at 7.6 and 10.1 cm were tested. Testing was performed in accordance with the ASME PTC 4.1 Power Test Code. The evaluation was conducted in the range of small- to mid-sized, natural gas-fired applications, from 1.06 to 2.64 X 10(6) kJ/h. The environmental nitrogen oxide (NOx) emissions performance was also compared for both burner technologies. The results of this evaluation indicate that firing the test boiler with the 7.6 cm new burner slightly improves unit thermal efficiency by 4.0-3.3% for a unit load ranging from approximately 1.06 to 1.79 X 10(6) kJ/h, respectively. Operation with the 10.1 cm new burner improves the thermal efficiency by approximately 1.6% at 2.32 X 10(6) kJ/h. The uncertainty in the measurements used in the efficiency calculations should account for approximately +/- 1.5% uncertainty in the reported gross efficiency. Reductions in NOx emissions of the order of 10% resulted from operation with the new premixed flame burners.