The effectiveness of bottom ash on the mechanical and physical properties of lightweight cement mortar for fire proofing application is investigated in this study. Bottom ash is well known that it can make it possible to decrease the thermal conduction in mortar by their porous structure. Physical properties of bottom ash including pozzolanic activity and compressive strength test were measured to decide the replacement amount in formulation as well as chemical composition. This study was undertaken on the use of bottom ash as fine aggregate in fire proofing mortar. Various dosage of bottom ash such as 25%, 50%, 75%, 100% were evaluated by several measurement. We found out there were serious correlation between specific gravity and thermal conductivity, so other porous materials were also investigated to decrease the thermal conductivity of cement mortar as well as bottom ash. In this study, the researches on the mixing proportion were mainly performed to design specification of spraying fire proofing mortar. Based on the laboratory test results, we’d like to suggest the proper adding amount of bottom ash by evaluation of consistency and strength development and then optimum mixing proportions of spraying fire proofing mortar using bottom ash by various evaluations.