This article presents a novel investigation on the Mo/Mg/Ni/O catalysts (Nmm-cats) with which the noncharring polyolefins are basically transferred to the charring polymers under forced flaming conditions and the flame retardancy of polyolefins is improved dramatically. The results of model charring experiments show that when appropriate Mo/Mg/Ni molar ratio is adopted and only 3% Nmm-cats is blended, the Nmm-cats belong to high-efficient charring catalysts that can deposit 56 and 64 wt % of volatile products in-situ from linear low-density polyethylene (LLDPE) and polypropylene (PP), respectively. The charring properties are characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy; and the char-forming mechanisms are analyzed by wide-angle X-ray diffraction experiment. The improvement in flammability properties for LLDPE and PP is demonstrated by using a cone calorimeter. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013