A power transmission apparatus with over-loading protection and power-saving mechanism is provided. The power transmission apparatus includes a switch module and a control module. The switch module includes a first switch circuit, a second switch circuit and a protection circuit. The first switch circuit is coupled between a power input module and a power supply port. The second switch circuit is coupled to the power input module. The protection circuit is coupled between the second switch circuit and the power supply port and detects a load power of the power supply port when the second switch circuit is turned-on. When the load power is greater than a predetermined over-loading threshold, the protection circuit enables the first switch circuit. After the control module determines that the first switch circuit is enabled, the control module controls the first switch circuit keeps enabling and disables the second switch circuit and the protection circuit.