This study proposes a green, low‐cost, and robust method to synthesize spherical calcium hydroxide (CH) nanoparticles based on the co‐precipitation of Ca²⁺ and OH⁻ in the presence of tannic acid (TA), an inexpensive plant‐derived polyphenol. It has been found that the morphology and crystalline phase of the produced CH nanoparticles can be controlled by TA. With the increasing dosage of TA, the crystallinity of the CH particles reduces, changing from well crystallization to amorphous phases. Experimental evidence suggests that complexion reaction took place between TA and Ca²⁺, which gave the Ca²⁺ a strong bond effect. As a result, the nucleation and growth of CH were significantly affected. CH nanoparticles with sizes of about 50 nm have been successfully synthesized by using 0.3% or more TA.