Calorimetric titrations have been performed in anthydrous acetonitrile at 298.15 K in order to obtain the complex stabillty constants(K_s) and the thermodynamic parameters for the complexation of rare earth(Ⅲ) nitrates (La, Ce, Pr, Nd, Sm, Eu) with 2,3-diphenyl-ene-(2)-1,4,7,10,13-pentaoxacyclopentadecane (Stilbeno-15-crown-5). The complexation stoichiometry is 1:1 for all rare earth(Ⅲ) nitrates. The complex stability constants (K_s), reaction enthalpies(△H~o) and entropies (△S~o) were calculated directly by using precision calorimeter connected to an personal microcomputer. The coordination of the unsaturated crown ether B (to see Fig.1) with Pr~(3+) showed the highest complexation ability and selectivity in the six kinds of rare earth (III) nitrates, and compared with the results of 15-crown-4A (in Fig.1). The thermodynamics quantities clearly indicate that the comples stability sequence is essentially entropy verned, although the complex formation itself is evidently enthalpy driven. The effects of unsaturated crown ether's molecular structure and cation's properties upon complex stability have been discussed from a viewpoint of thermodynamics.