  Line ratio of $3d-2p$ transition lines in boron-like spectra of Si X, S XII,
Ar XIV and Fe XXII has been investigated. Collisional-radiative model
calculations reveal that the line ratio is sensitive to the electron density in
ranges of $n_{\rm e}=4.0\times10^7-3.0\times10^{10}$ cm$^{-3}$,
$4.0\times10^8-3.0\times10^{11}$ cm$^{-3}$, $3.0\times10^9-4.0\times10^{12}$
cm$^{-3}$ and $2.0\times10^{12}-3.0\times10^{15}$ cm$^{-3}$, respectively. This
complements the K-shell diagnostics of helium-like ions. By comparison between
the prediction and the measured values, effective electron densities in the
electron beam ion trap (EBIT) plasmas performed by Lepson and collaborators at
Lawrence Livermore EBIT, are estimated to be $n_{\rm
e}=3.4^{+0.8}_{-0.6}\times10^{10}$ cm$^{-3}$ and
$5.6^{+1.0}_{-1.1}\times10^{10}$ cm$^{-3}$ for sulphur and argon plasmas. In
case of argon, a good agreement is shown with the actual electron density
derived from N VI K-shell spectrum. We further explore the $3d-2p$ transition
lines of Si X and S XII in the stellar coronal spectra measured with the Low
Energy Transmission Grating Spectrometer combined with High Resolution Camera
on board the {\it Chandra X-ray Observatory}. The constrained electron
densities show a good agreement with the those determined from C V and O VII
K-shell spectra.
