A ceiling fan blade fastening structure includes a base and a fan blade. The base is fixed to a rotor of a motor of a ceiling fan. The base has a bottom wall and a peripheral wall. The peripheral wall has a slot. The bottom wall is provided with a guide rail portion adjacent to the slot. The bottom wall is provided with a bracket and a positioning block behind the guide rail portion. The fan blade has a tongue portion to be inserted into the slot and pressed against the positioning block, so as to position the fan blade. The tongue portion is provided with a stop block to be against the rear of the bracket, so that the fan blade is unable to be disengaged from the slot.