A silicon physically unclonable function (PUF) using ring oscillators (ROs) has the advantage of easy application in both an application specific integrated circuit (ASIC) and a field-programmable gate array (FPGA). Here, we provide an RO-PUF using the initial waveform of the ROs based on 65nm CMOS technology. Compared with the conventional RO-PUF, the number of ROs is greatly reduced and the time needed to generate an ID is within a couple of system clocks.