An optoelectronic voltage sensor based on a timer chip and pulse durat ion ratio modulation is proposed and investigated experimentally in order to measure direct current (dc) high vol tage.The dc high voltage to be measured can be proportionally converted into the voltage sensing signals modula ted by pulse duration ratio by use of the ICM7555timer chip.The optical sensing signal is generated by a ligh t-emitting diode (LED) which is modulated by the above pulse signal,and transmitted to a photo-detector and si gnal processing circuits through a piece of plastic optical fiber.The dc voltage sensing signal to be measured can be demodulated from the measurement of the output pulse duration ration.The comparison analysis of rela tive errors has been conducted for the sensing schemes respectively based on pulse frequency modulation and pulse d uration ratio modulation,and relative errors are smaller for the duration ration modulation than those for th e frequency modulation.The dc high voltage with amplitude of 10kV has been measured experimentally,and the fiducia l error of the experimental data is less than 0.6%.