A simple odd number frequency divider with 50% duty cycle is presented. The odd number frequency divider consists of a general odd number counter and the proposed duty cycle trimming circuit. The duty cycle trimming circuit can output 50% duty cycle with only additional six transistors. A prototype divide-by-5 circuit with 50% duty cycle was implemented for a 500-Mb/s ∼ 5.6-Gb/s 1:10 CDR/DEMUX IC in a 0.13μm 1P8M CMOS process.