Korean bentonite was treated with aqueous NaOH solution under the reaction conditions such as concentration of NaOH, 0.5-6N; ratio of $Na_{2}O$ to $SiO_2$, 1-4; reaction time, 2-30 days; reaction temperature, $70^{circ}C-90^{circ}C$. The products were examined by X-ray diffraction patterns. When it was treated with 2N NaOH at $70^{circ}C$, zeolite species $P_1$ was formed with good yield. In higher concentration and at higher temperature than above, zeolite species $P_1$ was converted to hyeroxysodalite. Together with these crystals, some faujasite type zeolite, sodium A zeolite, mordenite type zeolite etc. was formed depending upon reaction conditions.