diff --git a/addons/purchase/purchase.py b/addons/purchase/purchase.py
index 10d8a34ba74..4aa59744a12 100644
--- a/addons/purchase/purchase.py
+++ b/addons/purchase/purchase.py
@@ -853,6 +853,14 @@ class purchase_order_line(osv.osv):
             res[line.id] = cur_obj.round(cr, uid, cur, taxes['total'])
         return res
 
+    def _get_uom_id(self, cr, uid, context=None):
+        try:
+            proxy = self.pool.get('ir.model.data')
+            result = proxy.get_object_reference(cr, uid, 'product', 'product_uom_unit')
+            return result[1]
+        except Exception, ex:
+            return False
+
     _columns = {
         'name': fields.text('Description', required=True),
         'product_qty': fields.float('Quantity', digits_compute=dp.get_precision('Product Unit of Measure'), required=True),
@@ -879,6 +887,7 @@ class purchase_order_line(osv.osv):
 
     }
     _defaults = {
+        'product_uom' : _get_uom_id,
         'product_qty': lambda *a: 1.0,
         'state': lambda *args: 'draft',
         'invoiced': lambda *a: 0,