Amylase is an important catalyst in the human digestive system. Plants are a source of this enzyme but there is no report of this enzyme for large Leucaena leaves or Krathin Kaset. The present study was divided into two parts of (1) detecting existing amylase using agar well diffusion, and (2) determining the optimal conditions of pH and temperature for enzyme activity of amylase in young large Leucaena leaves. Yellow clear supernatant was separated using a fruit extractor and used as a crude enzyme sample without dilution. Existing amylase in the enzyme sample was confirmed qualitatively by starch-agar well diffusion. The enzyme activity was determined, as a function of pH and temperature, using DNS colorimetric method. The optimum activity of the enzyme sample (9.54±0.17 U/g fresh weight) occurred at pH 5.8 and 40°C. Moreover, at 60°C, amylase worked best with starch solutions prepared in buffers pH 5.0 (8.24±0.00 U/g FW) and pH 9.0 (8.30±0.17 U/g FW). Similarly, enzyme activity of Leucaena amylase in buffer pH 7.8 (8.97±0.17 U/g FW) reached a maximum at 70°C. Large Leucaena leaves available anywhere in Thailand are proved as a useful teaching tool as a source of amylase from plant instead of that in human saliva, and commercial one.