  Decoherence often happens in the quantum world. We try to utilize quantum
dephasing to build an optimal thermometry. By calculating the
Cram$\acute{e}$r-Rao bound, we prove that the Ramsey measurement is the optimal
way to measure the temperature for uncorrelated particles. Using the optimal
measurement, the metrological equivalence of product and maximally entangled
states of initial quantum probes that always holds. However, using Ramsey
measurement, the metrological equivalence only holds in special situation.
Contrary to frequency estimation, the quantum limit can be surpassed under the
case $\nu<1$. For the general Zeno regime($\nu=2$), uncorrelated product states
are the optimal choose in typical Ramsey spectroscopy set-up. In order to
surpass the standard scaling, we propose to change the interaction strength
with time. Finally, we investigate other environmental influences on the
measurement precision of temperature. Base on it, we define a new way to
measure non-Markovian effect.
