The aim of this study was to examine the particle length, width, perimeter, and aspect ratio of calcium hydroxide powder using a flow particle image analyzer (FPIA). Five sample groups each with 10 mg of calcium hydroxide were mixed with 15 mL of alcohol and sonicated. Digital images of the particle samples were taken using the FPIA and analyzed with a one-way analysis of variance. The overall averages ± standard deviation among the five groups for particle length (μm), width (μm), perimeter (μm), and aspect ratio were 2.255 ± 1.994, 1.620 ± 1.464, 6.699 ± 5.598, and 0.737 ± 0.149, respectively. No statistical significance was observed among the groups for all parameters. When the total of 46,818 particles from all five groups were classified into the five length categories of 0.5-μm increments, there were significant differences in width, perimeter, and aspect ratio (all p values <0.0001). In conclusion, calcium hydroxide particles have a size and shape that may allow direct penetration into open dentin tubules.