An object of the present invention is to provide a hydrophilic cellulose derivative fine particle having a small particle size, a dispersion liquid thereof and a dispersion body thereof; and provide a diagnostic reagent composed of the hydrophilic particle, which is excellent in storage stability and does not require excess components, such as an emulsifier or surfactant. The cellulose derivative fine particle of the present invention is a cellulose derivative fine particle comprising a cellulose derivative with a part of hydroxyl groups of cellulose being substituted with a substituent, wherein the average particle diameter is from 9 to 1,000 nm; and the diagnostic reagent of the present invention is a diagnostic reagent obtained by loading a substance differentially interacting with a test object substance on the above-described cellulose derivative fine particle.