Reversible thermochromisms in the aqueous solutions of the lanthanide chelates with xylenol orange have been investigated by visual observation and spectro-photometry at pH 4.0. The color change factor Fc，defined as F_c=|⊿A₁十⊿A₂|， is suggested for characterizing the thermochromic extent of the chelates. The relations of the F_c value with the number of lanthanide elements，the ionic radii and the number of 4f-electron of the Ln(Ⅲ)-ions，which shows remarkable "double W" effect，have been studied and explained by"hole theory".