[en] The conditions for accurate electron density diagnostics in the solar transition region are discussed, and result shows that lines from Si VIII can provide an excellent tool for electron density diagnostics of the emitting plasma. For the Si VIII 1440.50 Å and 1445.75 Å lines, the principle of the electron density diagnostics is discussed for any intensity ratio. By the observed intensity ratio, the diagnostic results of the electron density for the quiet sun and the active region are calculated, and results indicate that in the quiet sun, the averaged electron density is $\log <!--\; ???\; -->(N_e)=8.63$; while in the active region, the averaged density gets the maximum $\log <!--\; ???\; -->(N_e)=8.86$ in the active region (B), and gets the minimum $\log <!--\; ???\; -->(N_e)=8.38$ in the active region (E), where the electron density is in the unit of cm⁻³. Finally, the relationship of intensity ratio and electron density is discussed, in the case of lower and higher electron density limits. This discussion is significant in the electron density diagnostics, which will be important for study on coronal heating and acceleration of solar wind.