Abstract: A toilet seat attachment structure is used for attaching a toilet seat unit on a toilet bowl. The structure includes a bolt with an axis portion, an abutting member disposed around the axis portion, an expanding member situated adjacent to the abutting member, and a nut member situated adjacent to the expanding member and engaging the axis portion. The abutting member has a shaft portion with a through-hole, and a flange formed on an upper periphery of the shaft portion. The axis portion passes through the abutting member and the expanding member, and engages the nut member. A friction member is disposed outside the expanding member. When the bolt is rotated, the nut member ascends on the axis portion, so that the expanding member gradually expands the diameter. Accordingly, the frictional member is enlarged to frictionally engage with an attachment hole of a toilet bowl.