Using the contact potential difference method, we have designed and established a measurement system for work function,which consists of three main parts such as,signal generation part, vibration part and detection part. The reference electrode is vibrated by a low frequency sine wave signal provided by signal generation part. When the output signal of de tection part is zero, by changing the load voltage of vibration part, we can get the work function of a sample by calclating the difference between the load voltage and the reference electrode. A change in the work function of ITO is observed when it is treated by UV ozone. The investigations indicate that the work function of ITO has a limiting value and the improvement in the work function of ITO can improve the performance of OLEDs. This system can measure the work function precisely accurately and quickly in atmosphere.