Decoherence often happens in the quantum world. We try to utilize quantum dephasing to build an optimal thermometry. By calculating the Cramér–Rao bound, we prove that the Ramsey measurement is the optimal way to measure the temperature for uncorrelated probe particles. Using the optimal measurement, the metrological equivalence of product and maximally entangled state of initial quantum probes always holds. Contrary to frequency estimation, the optimal temperature estimation can be obtained in the case ν<1\documentclass[12pt]{minimal}
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\begin{document}$$\nu <1$$\end{document}, not ν>1\documentclass[12pt]{minimal}
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\begin{document}$$\nu >1$$\end{document}. For the general Zeno regime (ν=2\documentclass[12pt]{minimal}
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\begin{document}$$\nu =2$$\end{document}), uncorrelated product states are the optimal choice in typical Ramsey spectroscopy setup. In order to improve the resolution of temperature, one should reduce the characteristic time of dephasing factor γ(t)∝t2\documentclass[12pt]{minimal}
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\begin{document}$$\gamma (t)\propto t^2$$\end{document}, and the power ν<1\documentclass[12pt]{minimal}
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\begin{document}$$\nu <1$$\end{document} appears after it. Under the imperfect condition, maximally entangled state can perform better than product state. Finally, we investigate other environmental influence on the measurement precision of temperature. Based on it, we define a new way to measure non-Markovian effect.
