Internal bias electric field in the lithium niobate (LN) crystal has a direct impact to the ferroelectric, electro-optic, and nonlinear effects of the crystal and the associated applications. A method to measure this field was proposed, and such a measurement in congruent lithium niobate (CLN) crystals, near-stoichiometric lithium niobate (nSLN) crystals, and doped LN crystals was performed. The results show that the internal bias electric field in the CLN crystal (reaching 2.53 kV/mm) is the largest among the three cases. Compared with the CLN crystal, this field greatly reduces in the nSLN crystals, and can be even lower by two orders of magnitudes for those grown by the lithium-rich melts method followed by vapor transport equilibration (VTE) treatment. In the doped LN crystals, the internal bias electric fields for the cases of 6.5% (mole fraction) Mg and 7% (mole fraction) Zn doping are 4 and 6 times smaller than that in the CLN crystal, respectively. The reason for causing the difference in the two doped cases was briefly discussed.