In　this　paper,　a　computer　program,　Drum　Boiler　Start-up　Simulation　Program　(DBSSP),　is　developed　for　simulating　the　start　up　behavior　of　controlled　circulation　and　natural　circulation　boilers.　The　mathematical　model　developed　here　is　based　on　the　first　principles　of　mass,　energy　and　momentum　conservations.　In　the　boiler　model,　heat　transfer　in　the　waterwall,　the　superheater,　the　reheater　and　the　economizer　Is　simulated　by　the　distributing　parameter　method,　while　heat　transfer　in　the　drum　and　the　downcomer　is　simulated　by　lumped　parameter　analysis.　The　program　can　provide　detailed　flow　and　thermodynamic　characteristics　of　the　boiler　components.　The　development　of　this　program　is　based　only　on　design　data,　so　it　can　be　used　for　any　subcritical,　controlled　or　natural　circulation　boiler.　The　simulation　results　were　compared　with　experimental　measurements,　and　good　agreements　between　them　were　found.　This　program　is　expected　to　be　useful　for　predicting　the　characteristics　and　the　performance　of　controlled　circulation　and　natural　circulation　boilers　during　the　start　up　process.　It　also　can　be　used　to　optimize　a　start　up　system　for　minimum　start　up　time.　(C)　2004　Elsevier　Ltd.　All　rights　reserved.