A circuit for trimming an internal oscillator of a USB device that generates a clock signal as a frequency source of the USB device includes a counter, a first detector for detecting an end of packet from an input data stream to initialize a counter, a second detector for detecting a synchronization sequence, a token packet or a handshake packet in the data stream for the counter to carry out clock counting on the clock signal, and a trimming code controller for comparing the count value with a reference value to determine a trimming code for trimming a clock frequency of the internal oscillator.