In this study, recycled PET (r-PET) flakes were blended with pyromellitic dianhydride (PMDA) and poly(styrene-co-glycidyl methacrylate-co-methyl methacrylate) (ADR-4400) as chain extenders in the melt state at varying amounts, reaction times and temperatures. The rheological and mechanical properties of the r-PET/PMDA and r-PET/ADR-4400 were analyzed to confirm the effect of the reaction conditions. The molecular weight of recycled PET flakes increases with the addition of chain extenders and it was confirmed that PMDA, a low molecular weight chain extender, increased the molecular weight more than ADR-4400, a high molecular weight chain extender. The tensile strength of recycled PET increases due to the chain extension reaction, with a maximum value at an addition level of 0.5 phr. The effect of reaction temperature on tensile strength shows that ADR-4400 has a maximum value at 270 ℃, while PMDA tends to decrease with increasing reaction temperature. For reaction time, ADR-4400 shows a maximum value at 5 minutes, while PMDA shows an increase with longer reaction time. The initial modulus is lower for ADR-4400 than for PMDA, which is attributed to the decrease in crystallinity and the change in entropy. A concentration of 0.5 phr, a reaction temperature of 270 °C, and a reaction time of 5 minutes were found to be the optimal conditions. Both PMDA and ADR-4400 in a 5:5 mixing ratio can be used to improve tensile strength and minimize the decrease in Young’s modulus.