The　coupled　effect　of　wall　heat　loss　and　viscosity　friction　on　flame　propagation　and　deflagration　to　detonation　transition　(DDT)　in　micro-scale　channel　is　investigated　by　high-resolution　numerical　simulations.　The　results　show　that　when　the　heat　loss　at　walls　is　considered,　the　oscillating　flame　presents　a　reciprocating　motion　of　the　flame　front.　The　channel　width　and　Boit　number　are　varied　to　understand　the　effect　of　heat　loss　on　the　oscillating　flame　and　DDT.　It　is　found　that　the　oscillating　propagation　is　determined　by　the　competition　between　wall　heat　loss　and　viscous　friction.　The　flame　retreat　is　led　by　the　adverse　pressure　gradient　caused　by　thermal　contraction,　while　it　is　inhibited　by　the　viscous　effects　of　wall　friction　and　flame　boundary　layer.　The　adverse　pressure　gradient　formed　in　front　of　a　flame,　caused　by　the　heat　loss　and　thermal　contraction,　is　the　main　reason　for　the　flame　retreat.　Furthermore,　the　oscillating　flame　can　develop　to　a　detonation　due　to　the　pressure　rise　by　thermal　expansion　and　wall　friction.　The　transition　to　detonation　depends　non-monotonically　on　the　channel　width.