  We study the electromagnetic Dalitz (EM) decays, $\psi \to \eta_{c} l^{+}
l^{-}$ and $\Upsilon \to \eta_{b} l^{+} l^{-}$ ($l = e$ or $\mu$), in which the
lepton pair comes from the virtual photon emitted by the M1 transition from
$c\bar{c}$ ($b\bar{b}$) spin triplet state to the spin singlet state. We
estimate the partial width of $\psi(\Upsilon) \to \eta_{c}(\eta_{b}) l^{+}
l^{-}$, based on the simple pole approximation. Besides, based on different QCD
models, the partial width of $\psi(\Upsilon) \to \eta_{c}(\eta_{b}) \gamma$ is
determined.
