Polyacrylamide gel system is widely been used in oilfield for water shutoff treatment. Gelation behavior such as proper gelation time provides a basic guideline on gel placement depth in reservoir layers. In this study, the gelation behavior of partially hydrolyzed polyacrylamide crosslinked with chromium(III) acetate (PHPA/Cr3+) reinforced with nano-silica was studied in term of gelation time and gel stability. The gelation time of PHPA/Cr3+ gel could be controlled with the addition of nano-silica where the gel has shown a delay in gelation time at low concentration of nano-silica. At 0.3 wt. of nano-silica concentration, the gelation time has shown two times delay compared to the original gel system. The gel at the formulations of 0.7 wt. and 1.0 wt. nano-silica content remained stable with H code after 24 hours observation. © 2020 Trans Tech Publications Ltd, Switzerland.