Phase noise of oscillators can be improved through noise filtering. This paper presents a transformer-based harmonic filtering technique to improve the phase noise performance of the fully differential CMOS LC oscillators. The primary and secondary of the transformer are placed at the sources of NMOS and PMOS differential pairs, respectively. Due to the extra degree of freedom introduced by the transformer, the filter can resonate at two different frequencies, which are designed to be the second and fourth harmonics. The analysis of the filtering method is presented and principles of operation are discussed in details. The performance is compared with other filtering techniques, and the simulation results show improved FoM compared to published literature.