A/D converter (ADC) is a basic device in digital signal processing systems. For high-speed applications, a flash ADC type is often used. Due to require many analog comparators, the chip complexity and power dissipation become very high. Moreover, the accuracy of dividing resistors requires very high for reference voltage if the converting resolution is high. In this study, we develop a new kind of flash ADC based on a simple CMOS circuit. By adjusting the ratio of channel length and width, the transition threshold of the CMOS inverters is various to detect input analog signal. Then their results are encoded to the digital code. The advantages are that the ADC circuit does not need any resistor and use simple CMOS inverters rather than analog comparators. The new 8-bit ADC chip only used 634 transistors. The power dissipates 0.9mW using 0.35um process when it operates at 100MHz.