Abstract: In order to modify the nano-sized calcium carbonate better，fatty acid was adopted to dope it in-situ and the mechanism of in-situ encapsulation was discussed.An appropriate amount of alkali was introduced to some just prepared milk of nano-sized calcium carbonate which followed by being heated to 75.0~90.0 ℃.By the aid of stirring，needed amount of fatty acid was added into the milk.After filtrating，drying，and pulverizing，nano-sized encapsulated calcium carbonate powders were obtained.Characterization of oil absorption，contact angles，and scanning electron microscopy of the powders，and their filling application experiments in vulcanized silicone rubber and DOP in room temperature  indicated: each calcium carbonate particle was uniformly and thoroughly encapsulated by fatty acid，and the calcium carbonate fillers could drastically enhance some properties of polymer.Research showed that  90 ℃，n（SA+OA）∶n（OH^-）=1∶1 and n（SA+OA）/m（CaCO₃）=1.0×10^-4 mol/g were the suitable conditions for the in-situ encapsulation of nano-sized calcium carbonate.

Key words: nano-sized calcium carbonate, in-situ encapsulation, fatty acid