The invention provides a quadrature clock generating apparatus and a communication system transmitter. The quadrature clock generating apparatus connected to a local oscillator generating an input clock signal and an inverted input clock signal includes a fractional dividing circuit and a quadrature signal generating circuit. The fractional dividing circuit is configured for receiving the input clock signal and the inverted input clock signal, and for performing frequency-division upon the input clock signal and the inverted input clock signal to generate a frequency-divided clock signal according to a fractional dividing parameter. The quadrature signal generating circuit is configured for receiving the input clock signal, the inverted input clock signal, and the frequency-divided clock signal to generate a plurality of quadrature clock signals. A plurality of quadrature clock signals with quadrature phases can be generated to realize low power consumption.