Abstract: The stability constant of coordination compound of 18-crown-6 with La (Ⅲ) in methanol was determined using Na+ as the competing ion, Na+-selective electrode as the indicating eletrode and Ag-AgCl electrode as the reference.The electromotive forces of the cellAg-AgCI |0.1M (C2H5)4NCl, xMNa+, yMl8C6, zMLa3+| Na+-electrode were measured under different conditions, and concentrations of free ions were calculated using Nernst's equation, and then stability constants βMn+ were calculated to be log βna+=3.92±0.08, logβLa9+ =3.33±0.14. These values were in good agreement with values reported in the literature. This method may serve as a simple and convenient means for the systematic study on the stability of coordination compounds of f-group metal-ions with crown ethers.