Recently, mini-implant is popular in the orthodontic treatment due to its simplicity and convenient surgical procedure. The objective of this study is to provide the anatomical guideline for mini-implant placement by analysing the cortical bone thickness in Korean. Hemi-sections of sixteen maxillae and twenty-two mandibles with normal teeth were used. Interdental areas between the 1st premolar and the 2nd premolar (Group 1), the 2nd premolar and the 1st molar (Gruop 2), and the 1st molar and the 2nd molar (Group 3) were sectioned and then scanned. After setting the axis of teeth, the cortical bone thickness was measured at the distance of 2 mm, 4mm, 6 mm, and 8 mm from alveolar crest. The mean thickness of cortical bone in the maxilla according to distance from alveolar crest was $1.30;{pm};0.63; mm$ (2 mm), $1.49;{pm};0.62; mm$ (4mm), $1.72;{pm};0.64; mm$ (6mm), and $1.90;{pm};0.90; mm$ (8 mm) at the buccal side and $1.33;{pm};0.47 ;mm$, $1.31;{pm};0.45; mm$, $1.37;{pm};0.55; mm$, and $1.39;{pm};0.58 ;mm$ at the palatal side. In the mandible, that was $3.14;{pm};1.71 ;mm$, $4.31;{pm};2.22 ;mm$, $4.23;{pm};1.94 ;mm$, and $4.30;{pm};1.57; mm$ at the buccal side and $1.98;{pm};0.88 ;mm$, $2.79;{pm};1.01; mm$, $3.35;{pm};1.27$ mm, and $3.93;{pm};1.38; mm$ at the lingual side. The buccal cortical bone thickness in the maxilla was decreased from Group 1 to Group 3, while the thickness of palatal side was no change. In the mandible, it did not show a tendency at the buccal side and it was decreased from Group 1 to Group 3 without significant difference at the lingual side. Therefore, the buccal side of the Group 1 and Group 2 in both the maxilla and mandible seems to be the most appropriate site for a mini-implant placement with taking the stability and retention.