Abstract: Disclosed is a cleaning agent for heater tubes, which is capable of eliminating deposits such as soot particulates in tubes installed in industrial heaters that typically use oil, coal or gas. The cleaning agent of the present invention is prepared by formulating a chemical composition comprising ammonium nitrate into pellets. Also, the present invention discloses a method of cleaning heater tubes using the cleaning agent. When being utilized for cleaning the heater tubes, the cleaning agent can improve mechanical and chemical cleaning effects, thus shortening of working time and reducing a required amount of the chemical composition, as well as preventing rapid evaporation of the chemical composition, thus increasing cleaning effect. In addition, the cleaning agent can eliminate the cementing effect of sulfur oxides or vanadic oxides in the heater tubes and thus control stably emission of NOx, by selectively containing other additives including magnesium and urea.