A new cassane diterpenoid, caesaljapin, was isolated from Caesalpinia decapetala var. japonica and the structure was deduced from spectral data. The known compounds, lup20(29)-en-3fi-01, betulinic acid, 3-deoxysappanchalcone, sappanchalcone, catechin, methyl gallate and 3-hydroxy-l-(4-hydroxy-3-methoxyphenyl)-lpropanone were also isolated.