Abstract: The color reaction of trichloroarscnazo (TCA) (2-(2-arsonophcnylazo)-7-(2, 4,6-trichlorophcnylazo)-1,8-dihydroxy-3,6-disulfonaphthalcne) with rare earth has been studied in this paper.TCAwas found to have high sensitivity to Cerium group rare earth ε = 1.079-1.308×10⁵ and to the amount of rare earths ε = 0.937-1.329×10⁵), big comparison degree (Δλ = 93-104nm), and good selectivity. Large amounts of Ca (10mg), Mg(8mg), Al (60mg) do not interfere with the determination. It is one of the best selective reagents for rare earths up to now.