The characteristics of bed voidage in a fluidized bed of polyethylene particles polymerized by metallocene and Ziegler-Natta catalysts were investigated in a fluidized bed of 7 cm I.D. and 50 cm in height. The average bed voidage for the ranges of gas velocity from minimum fluidization velocity to turbulent fluidization velocity were 0.4-0.65 for PE with Metallocene catalyst and 0.4-0.85 for PE with Ziegler-Natta catalyst. The pressure fluctuations were smaller for PE with Ziegler-Natta catalyst than that with Metallocene catalyst. The addition of PE with Ziegler-Natta catalyst to the PE with Metallocene catalyst reduced the pressure fluctuations and increased the bed voidage.