A low power tunnel diode oscillator with high frequency stability, which operates at liquid helium temperatures is reported. The stability of the oscillating frequency is affected not only by the current flowing in the diode, but also the stability of temperatures of the circuit. Many attempts to stabilize the frequency are discussed here. One of the results of the density of He³-He⁴ mixtures (15% He³-He⁴), measured by this circuit, is also reported.