Fire is a complex chemical process and one of the most common causes of loss of property and life. Forensic investigator must understand the basic chemistry and physics of fire to know the exact origin and cause of it. The diffusion flame process consists of three basic elements: fuel, oxygen and heat. The six elements of the life cycle of fire are described by Dawson Powell in The Mechanics of Fire. These elements are input heat, fuel, oxygen, proportioning, mixing, and ignition continuity. All fires produce combustion products. Combustion products fall into four categories: heat, gases, flame, and smoke. A fire in a room or defined space generally will progress through three predictable developmental stages. In order to determine the origin and cause of a fire properly, the investigator must be able to interpret the effects of these stages correctly during the examination of the fire scene. In this paper, I discussed about the life cycle of fire, effect of various fire gases, flame colour, flame types and room fire sequence to provide a scientific basis for determination of origin and cause of fire for investigator.