Grape seeds are usually byproducts from wine and juice processes and they still contain abundant of bioactive compounds which are now being consumed for health promotion. In the present study, the effects of different extracting conditions such as grape seed byproduct sources (wine and juice processing), extracting methods (shaking and ultrasonic-assisted) and particle sizes (ground, <20, 20-40 and >40 mesh) were investigated. The results showed that the defatted particle of >40 mesh of the seeds from juice process using ultrasonic-assisted extraction contained higher amounts (P<0.05) of total phenolics and total flavonoids for 362.02 mg GAE/g DM and 272.61 mg CE/g DM, respectively and phenolic compounds namely catechin, procyanidin B2 and epicatechin were 12.60, 3.29 and 12.42 mg/g DM, respectively. The antioxidant activities as DPPH, FRAP and ABTS were also higher (P<0.05) for 2,583.77, 2,450.26 and 4,806.11 mmol TE/g DM, respectively. In addition, the total phenolic contents indicated highly positive correlation with antioxidant activity.