PURPOSE: To obtain the title aqueous suspension having a low viscosity and excellent storage stability and being useful as a polymerization initiator for an ethylenically unsaturated monomer, comprising a solid organic peroxide, a dispersant which is a specified water-soluble crosslinkable acrylic resin and water.

CONSTITUTION: 5W6wt% solid organic peroxide (A) (e.g., benzoyl peroxide), 0.001W5wt% dispersant (B) which is a water-soluble crosslinkable acrylic resin having a yield value of an aqueous solution at 20°C≥10 dyn/cm2 and an R value which is a ratio of the coefficient (P.g/cm.sec) of its aqueous solution to its yield value of 2W40sec, 0W10wt% surfactant (C), 0W20wt% low-MW monoalcohol, low-MW diol, etc. (D), 0W5wt% protective colloid (E) and, optionally, a dispersing aid, an alkali substance, etc., are dispersed in water (G).