This letter presents an implementation to reduce area occupation in designing voltage-controlled oscillators (VCOs) using a filtering technique. We applied a helical inductor to the noise filter in a 2.5-GHz CMOS VCO to reduce area occupation. Because a helical inductor has less area occupation, a small silicon area was achieved. This VCO operates in the 2.5-GHz band with power consumption of 1.5 mW and phase noise of -119.2 dBc/Hz at 1-MHz. Our VCO displays an excellent performance of phase noise in relation to power consumption.