[ $eta$ ]-Galactosidase was extracted from the internal organ of sea urchin, Hemicentrotus pulcherrimus The enzyme was purified 384.6-fold over the crude extract by the sequential chromatographic methods including DEAE-Sephadex A-25, CM-Cellulose, and Con A-Sepharose 4B affinity chromatography with a recovery $1.26\%$. The molecular weight of the purified enzyme was estimated approximately 94 kDa as monomeric term by SDS-PAGE and Sephadex G-150 gel chromatography. The maximum enzymatic activity was observed at pH 3.0 and $50^{circ}C$ but the one was stable over the ph range or 3.0$~$5.0 and below $37^{circ}C$. The $K_m$ and $V_{max}$ values against PNPG (P-nitrophenyl $eta$-D-galactopyranoside) were 15.0 mM and 214 $mu$mole/min per mg protein, respectively. The enzymatic activity was activated by $Ba^{2+}$, but significantly inhibited by $DEP,;Hg^{2+},;Sn^{2+}$ and galactose.