The present invention provides a modular system having a first connection structure, a fan unit, and a circuit board unit. The circuit board unit includes a control and communication circuit board respectively electrically connected to a power module circuit board and a driver module circuit board. The driver module circuit board is electrically connected to a fan unit load of the fan unit. The power module circuit board, the control and communication circuit board, and the driver module circuit board are respectively mounted in a first protection shell of the first connection structure or in a fan protection shell of the fan unit. The modular system allows implementations of different circuit board structures for a ceiling fan with or without a pendant light. The modular system allows a control circuit board to more conveniently fit into the ceiling fan with or without the pendant light.