The present invention relates to an alloy steel for a mold tool and a mold using the same.The steel for the mold tool comprises iron (Fe) as a main component, an amount of about 0.35 to 0.45 wt % of carbon (C), amount of about 0.80 to 1.20 wt % of silicon (Si), amount of about 0.20 to 0.50 wt % of manganese (Mn), amount of about 6.00 to 8.00 wt % of chromium (Cr), amount of about 1.50 to 3.00 wt % of molybdenum (Mo), and amount of about 0.80 to 1.20 wt % of vanadium (V) based on the total weight of the tool steel. Accordingly, the mold tool have improved physical properties at high temperature and extended life span.