Size distribution, zeta potential, and yield of surfactant-free oil-in-water (O/W) emulsion prepared with oleic acid were measured at various pHs after seven days from the emulsification. We found that the size distribution is almost the same regardless of pH from 9 to 3. The magnitude of zeta potential and yield remains constant at pH from 9 to 6, and decreases at pH from 6 to 3. There is a correlation between the zeta potential and the yield, but no correlation between the zeta potential and the size, suggesting that the zeta potential relates to stability of oleic acid droplets in water rather than their size.