Hand-sewing (HS) and stapling are common parenchymal closure techniques after distal pancreatectomy. However, these methods cannot completely prevent postoperative pancreatic fistula (POPF). The mechanisms of POPF formation after closure are unknown. We performed distal pancreatectomy in mongrel dogs to identify the mechanisms of POPF formation after HS and staple closure. We measured the closed pancreatic duct burst pressures and examined the histology of the remnant pancreas. The after staple-closure burst pressures depended on stapler height; lower pressures were associated with greater stapler heights. Post-HS closure burst pressures were significantly higher than those at each stapler height (P<0.01). Post-HS closure pathologic findings showed extensive necrosis (day 3), and some regenerated pancreatic duct stumps (day 5). Necrosis was not observed around the stapled tissues. Although HS completely closes the pancreatic ducts, stump necrosis and blood flow disturbances may cause POPF. With stapler closure, pancreatic fluid leakage may occur even with appropriate stapler heights.