In fracturing operations, timingof gel breaking plays a great role in the enhancement of oil recovery. A poly(vinyl acetateacrylamide) (P(Vac-AA))core-shell microsphere coated with ammonium persulfate ((NH 4 ) 2 S 2 O 8 )has been synthesized as a gel breaker through inverse emulsion polymerization. Laboratory experiments have been done to compare the changes of the viscosity and pH value of polyacrylamide solution when the microsphere and ammonium persulfate were used as gel breaker respectively. Experimental results showed that the viscosity half-life of polyacrylamide solution, which was only several minutes when ammonium persulfate was directly used as gel breaker, was extended to 1 h when the microsphere was used as gel breaker. Meanwhile, the use of the P(Vac-AA) microsphere also slowed down the pH decrease of the polyacrylamide solution and reduced the magnitude of the pH decrease, indicating that the core-shell structure inhibited the rate of hydrogen ion generation by oxidative degradation of polyacrylamide in the solution. It is thus understood that by delaying its swelling action, the microsphere slowly releases ammonium persulfate, reducing the free radical oxidative degradation rate of polyacrylamide in aqueous solution. © 2018, The Editorial Board of Drilling Fluid & Completion Fluid. All right reserved.