The invention provides display control circuits for Vacuum Fluorescent Displays (VFDs). The display control circuit controls a plurality of display units of the VFD and comprises an image signal generator generating a plurality of image signals, a clock signal generator generating a clock signal, and a plurality of control signal generators. Each control signal generator receives one of the image signals and the clock signal, generates a control signal for one of the display unit, and determines the duty cycle of the control signal according to the received image signal and the clock signal. The brightness of one display unit varies with the duty cycles of the corresponding control signal. The clock signal generator comprises a plurality of flip-flops coupled in series and a plurality of logic gates.