Red phosphorus flame retardants for synthetic resin are produced by a method comprising the steps of heating yellow phosphorus at temperatures of 250 to 600 °C to effect a partial conversion of yellow phosphorus to red phosphorus; removing unconverted yellow phosphorus; and coating particles of the resulting red phosphorus with at least one of thermosetting resin and metal hydroxide. The thus obtained red phosphorus flame retardants are very stable because of significantly improved heat resistance, water resistance and weatherability and thus can provide nonflammable resinous compositions which can be used for long period of time. The nonflammable composition consists essentially of 100 parts by weight of polyolefine resin, 20 to 200 parts by weight of hydrated inorganic filler and 0.1 to 30 parts by weight of the coated red phosphorus flame retardant.