Plant lipases have been chiefly studied as an esterase for hydrolyzation of triacylglycerol (a true lipase), which supplies energy for seed germination. Lipases are widely distributed in plants, animals, insects, and microorganisms. However, recent studies suggest that plant lipases have physiological functions other than triacylglycerol hydrolysis. In the present study, a plant lipase that has enzyme properties distinct from those of a true lipase was purified and characterized from oat seedlings. The lipase was purified 189-fold to a 0.53% purification ratio with high specific activity (34.656 U/mg). Analysis of the protein by Sodium dodecyl sulfate polyacrylamide gel electrophoresis showed a homogenous purified lipase. The lipase had higher enzyme specificity to monoacylglyceride and short chain fatty acids. Synthesis of the lipase was active at an early stage of germination for 6 days and decreased thereafter. Most of the lipase was found in the upper part of the oat seedling excluding the root. Within the young leaves, the lipase is located only in vessels and sieve tubes. However, infection of a pathogen, Pseudomonas syrinae pv. oryzae, elevated the lipase synthesis. In addition, the lipase had an ability to hydrolyze E.coli lipopolysaccharide. These results suggested that oat lipase may play a physiological role in defense against pathogens.