In order to exactly reflect the effect of the radiant tube size on the combustion performance,thermal parameters on uncork-cylinder radiant tube gas-fired combustor were measured using different radiant tubes under three kinds of loads,and the relatively suitable matching relationship among the load,the length and the diameter of radiant tube was obtained,thus the optimization design on the combustor structure was achieved.It is found that 219 mm is the appropriate radiant tube diameter in the test load range,950 mm,1000 mm and 1100 mm are the appropriate radiant tube lengths under loads of 50 kW,75 kW and 100 kW.