This paper describes the new mixing method of silica onto "green" natural rubber (NR) using the sol-gel reaction of tetraethoxysilane (TEOS) before curing. NR was subject to swelling by TEOS and the sol-gel reaction of TEOS in NR matrix was conducted by a base catalysis to obtain NR/in situ silica mix. The resulted NR/in situ silica mix was compounded with curing reagents, followed by sulfur vulcanisation. Properties of the vulcanisates were investigated in comparison with those of carbon black stocks. The in situ silica stock was superior to the conventional silica stock, and similar to the carbon black stocks in term of physical properties. This difference between the two silicas is tentatively ascribed to the smaller concentration of silanol group on the surface of in situ silica, which results in less filler-to-filler interaction.