Four different sizes of tripolyphosphate modified expandable graphite (EG(p)) are prepared and incorporated into linear low density polyethylene (LLDPE). Its size effect on the material combustibility, thermal stability, and mechanical properties are investigated, respectively. The interactions between each EG(p) and ammonium polyphosphate (APP II) are also examined. Results confirm that EG(p) can significantly improve the flame retardancy and thermal stability of LLDPE composites than the normal expandable graphite. A 13% dose of EG(P50) can ensure that 87LLDPE/13EG(P50) meets the flame retardant requirement. There is a positive correlation between EG(P) size and LOI and high temperature thermal stability. Especially, the combination of EG(p) and APP produces a significant synergistic effect in improving the LOI value and the vertical burning level. The peak value of heat release rate of 87LLDPE/8.7EG(P50)/4.3APP can be remarkably reduced by 83.9%. However, addition of the tested additives causes a decline of LLDPE mechanical performance. With the size of EG(P) increases, the effect becomes more obvious.