This study determined the appropriate pressure for high pressure shucking of oysters (Magallana gigas) according to size categories. One-year and two-year-old oysters harvested from Yeosu and Tongyeong coastal areas were subjected to high pressure treatments with pressure ranging from 180 to 240 MPa and holding times of 0 to 420 seconds. Results demonstrated that 100% shucking rates were achieved at 220 MPa with 120-180 seconds holding time regardless of harvest time and production area. K-means cluster analysis was used to classify oyster specimens into small, medium, and large size groups with mean shell heights of 69.0 mm, 86.6 mm, and 106.7 mm, respectively. The analysis revealed that shucking efficiency improved directly proportional to oyster size. However, the appropriate shucking pressures converged at approximately 220 MPa for all size groups, with small, medium, and large groups showing 218.9 MPa, 221.7 MPa, and 220.6 MPa, respectively, and no significant differences among groups. These findings suggest that the appropriate pressure for high pressure shucking processes be approximately 220 MPa regardless of harvest time and production area, providing foundational data for potential automation of oyster shucking processes.