sParametric resonance based mass sensing leads to increased sensitivity over other resonant methods. In this work, we present a frequency-tuning approach to measure mass change in an ultra-sensitive mass sensor. This scheme drives the oscillator using an electrical signal with fixed frequency and tunes the parametric resonance frequency to match the driving frequency by feeding back a dc offset to the sensor. Instead of monitoring frequency shift of the oscillation in a micro-oscillator, mass change in the sensor can be detected by measuring the shift in de offset, making the sensor amenable to closed-loop control. This scheme and the prototype mass sensor with a mass of similar to 30 ng and a first mode harmonic resonance frequency of similar to 83 kHz has been built and experimentally characterized. Good correlation between parametric resonance frequency and dc offset has been shown and the noise floor is less than 1 pg. The sensor is used to track water content variation in a humidity-testing chamber and shows increased efficiency over open loop operation. (c) 2005 American Vacuum Society.