We have theoretically and experimentally proven that the transient transmitted laser power through a typical Λ system atomic cell near Coherent Population Trapping resonance presents as a damping oscillation. The oscillating frequency is identical to the frequency detuning from the atomic hyperfine splitting. Based on the transient coherent population trapping (TCPT) phenomenon, we have proposed an atomic clock in which the standard frequency is obtained by compensated the TCPT oscillating frequency to the RF modulating frequency. Therefore we do not need to lock the RF frequency. There will be advantages for atomic clock miniaturization and stabilization.In the past 10 years, the research on Coherent Population Trapping (CPT) atomic clock has received increasingly more attention, because it can be miniaturized [1]- [5]. Current researchers hope to utilize this in creating chip scale atomic clocks (CSAC). For the same purpose, we proposed a new scheme to implement an atomic clock based on the transient coherent population trapping (TCPT) phenomenon. We have proven that the transient transmitted laser power through a typical Ʌ system atomic cell, near the coherent population trapping resonance, presents as a damping oscillation. The oscillating frequency is equal to the frequency difference of the two pump lasers detuning from the atomic ground level hyperfine splitting. Therefore, we can directly measure the frequency detuning and then compensate to the output frequency of the microwave oscillator, which is used to modulate the pump laser. Then we can get the standard frequency exactly equal to the atomic ground level hyperfine splitting. By this method, we can further simplify the structure of the CPT atomic clock, and make it more stable, more reliable, and easily digitized and miniaturized.In our proposal, the atomic clock bases on the TCPT phenomenon, which happened when the two pump lasers' frequency difference have a small detuning from the CPT requested. Figure.1 shows a typical Ʌ system, such as D1 line of 85 Rb [6]-[7], with the two hyperfine ground-states |1> and |2>, as well as the excited state |3>. Γ is the decay rate from the excited state to the two ground levels considering the effects of both spontaneous emissions and collisions with buffer gas atoms [8]. Figure 1. Three-level system considered. State |3> corresponds to the first excited state of the alkali atom.