In this report, the following matters are explained from the result of our boiler test : 1. As the efficiency-load curves which have hitherto been used as the characteristic curves are not satisfactory, we recommend to use efficiency-load-excess air ratio surface. 2. By our testing method, we can accomplish the boiler test at high accuracy within about an hour, including the pre-operation to uniformize the combustion. 3. Boiler efficiency is a function of load, excess air ratio, draft, temperature of the flue gas, damper opening etc. two of which are independent variables. Hence we can operate the boiler at the state of maximum efficiency for a given load, when we observe any one of the other variables for example draft or damper opening.