A bicycle seat post includes two arcuate plates and a reinforcement board sandwiched between the two arcuate plates. Each arcuate plate has two lengthwise end surfaces and each end surface includes an inner corner edge and an elongate surface defined in an outer corner edge thereof. The two lengthwise end surfaces contact two opposite sides of the reinforcement board. Each of lengthwise top surface and lengthwise bottom surface of the reinforcement board is connected between the two respective elongate surfaces of the two arcuate plates so as to form two longitudinal grooves in an outer surface of the seat post.