Developing green and high efficiency inorganic flame retardants is the trend of preparing flame retarding polymer composites. Aluminum phosphates (t-hAP) with uniform, small dimension, and hexagonal structure were facilely synthesized, which have similar size (1–2 μm) but different structures from commercial spherical-like aluminum phosphate (cAP). The flame retardance of bismaleimide (BD)/t-hAP and BD/cAP composites were intensively investigated. t-hAP is proved to have much better flame retarding effect than cAP, but also exhibits advantages over Mg(OH)2 and Al(OH)3. With only 5 wt % addition of t-hAP into BD resin, the peak and total heat releases as well as total smoke production significantly reduce 42.3, 47.8, and 67.3%, respectively; besides, better data are obtained as the loading of t-hAP increases to 10 wt %. These attractive data result from three effects induced by t-hAP. Besides the better protection role of sheet structure, the strong hydrogen bonding between t-hAP and BD resin endows the composite with good dispersion of t-hAP and high crosslinking density; moreover, t-hAP releases H2O and NH3, diluting flammable gases during combustion. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41089.