High-density carbon blocks have excellent mechanical, thermal, and electrical properties. In particular, these blocks are applied in various fields while maintaining excellent physical properties even in harsh environments. In this study, binderless coke manufactured under air conditions to make O-functional groups on surface. And it was used to form green bodies (GBs) under various pressure conditions of 50 to 250 MPa, and the bodies were carbonized to form a high-density carbon blocks (CBs). Then, the effect of the β-resin and oxygen functional groups on the mechanical properties of the high-density carbon blocks depending on forming pressure was considered. When forming at a low pressure of under 200 MPa, the ratio of O and C (O/C) has a greater effect, and the higher the O/C, the higher the mechanical properties. On the other hand, when forming at a high pressure of 250 MPa, the β-resin content has a greater effect and steadily increases. When the β-resin content is low, the mechanical properties are decreased. That phenomena can be explained by porosity. In case of CB-N0A10-250 which has the lowest β-resin content of 0.4 wt%, porosity is 6.7%. However, in the case of CB-N7A3–250 which has the highest β-resin content of 3.7 wt%, porosity is 0.6%.