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The rapid growth of demand for wireless transmission has placed great pressure on the scarce radio spectrum due to the fixed spectrum allocation policy and cognitive radio (CR) is considered as a promising solution to such the problem. The main idea behind a cognitive radio network is for the unlicensed users (also called secondary users) to exploit opportunistically the underutilized spectrum licensed to a licensed network (referred to as a primary network). Before investigating the technical and political implications of CR, it is important to know to what extent the licensed bands are temporally unoccupied. In this paper, for the first time, we investigate the spectrum usage pattern in Vietnam in the frequency bands ranging from 20 MHz to 3000 MHz in Ho Chi Minh City and Long An province. The purpose of the measurement is to find how the scarce radio spectrum allocated to different services is utilized in Vietnam and identify the bands that could be accessed for future opportunistic use due to their low or no active utilization. These analyses indicate that, on average, the actual spectral usage in all bands is 13.74% and 11.19% for Ho Chi Minh City and Long An province, respectively. The experiment results also show that the spectrum band assigned for analog television (470-806) is the highest occupancy band with 58%.