  In this talk, recent measurements of charmonium decays of BESIII are
presented. Using 448 million $\psi(3686)$ events collected with the BESIII
detector, the branching fractions of the decays $\chi_{cJ} \to \phi \phi
(J=0,1,2)$ have been measured most precisely, and the polarization parameters
of $\chi_{cJ} \to \phi \phi$ have been determined for the first time via a
helicity amplitude analysis. Using the same data sample as in the previous
study, first evidence of $\eta_c(2S) \to \pi^+ \pi^- \eta$ has been found in
the decay sequence $\psi(3686) \to \gamma \eta_c(2S)(\to \pi^+ \pi^- \eta)$.
The product of the branching fractions of $\psi(3686) \to \gamma \eta_c(2S)$
and $\eta_c(2S) \to \pi^+ \pi^- \eta$ is reported as well as the individual
branching fraction of $\eta_c(2S) \to \pi^+ \pi^- \eta$. The process $e^+ e^-
\to \eta J/\psi$ at a center-of-mass energy $3.773~{\rm GeV}$ is observed for
the first time. Its Born cross-section is measured, and the branching fraction
of $\psi(3770) \to \eta J/\psi$ is determined by a combined fit with the cross
sections at other energy points, after considering the interference effect for
the first time. Utilizing 2708 million $\psi(3686)$ events collected by the
BESIII detector, the decays $\chi_{cJ} \to \Omega^- \bar{\Omega}^+(J=0,1,2)$
have been observed for the first time with high significance via the radiative
decays of $\psi(3686) \to \gamma \chi_{cJ}$. The relevant branching fractions
have been provided.
