The most concerned issue nowadays for the refrigeration circles is to find out a certain refrigerant that could tolerate with nature, also have excellent performance and is liable to obtain as well. In the light of safety of natural refrigerants, we set up an experimental apparatus for flammability limits and analyze the experimental principle. The reasons that may influence the value of the limits are given. The experimental investigation of the flammability limits of Propane, Isobutane and Methylamine is carried out. It is shown that, if the consistency of the refrigerants in air increases approximately to the flammability limits, high-energetic ignition source is also needed to explode. However, it is necessary to take some precautions for safety.