Value-added utilization of fly ash has recently gained a strong interest. As a solid waste, fly ash can be used as a paper filler, and the recent innovation on the production of high-brightness fly ash products further facilitated;rich applications. This work reports the results on using the novel fly ash based fillers in the paper making process, with a focus on the effect of filler particle size. In comparison with ground calcium carbonate (GCC) commonly used as paper fillers, the original fly ash based calcium silicate filler (FACS) has a larger particle size (27.6 mu m), a much lower true density (1.3-1.4 g/cm(3)), a higher specific surface area (121 m(2)/g), and a similar brightness (91% ISO). FACS exhibits porous, aggregated, and needle-like morphologies based on the results of scanning electron microscope image analysis. Ball milling decreased the particle size, broadened the particle size distribution, and improved the brightness while changing its morphology. The paper bulk increased dramatically when the original FACS was used due to its large particle size and narrow particle size distribution. With ball milling, the paper bulk and porosity decreased with decreasing particle size at the same filler content, while the tensile index increased. In addition, the ball milled FACS-filled paper had better light scattering coefficient and brightness than the GCC-filled piper.