The present invention relates to an inner fin 5 for heat exchanger flat tubes 22A which comprises a metal plate having a corrugated cross section and which is to be enclosed in the flat tube 22A, all ridges 51 and all furrows 52 of the inner fin being joinable to an inner surface of the flat tube 22A by brazing. At least one of the ridges 51 and the furrows 52 of the inner fin 5 is provided with a turbulence promoting protrusion 53 facing toward a side opposite to the side to be brazed. The turbulence promoting protrusion 53 is so sized that a clearance to be created between a caved portion 54 formed on a rear side of the protrusion and the inner surface of the flat tube 22A can be filled with a brazing material 6. A fluid to be passed through the flat tube 22A is caused to flow in a more turbulent state by the protrusion 53 to thereby achieve an improved heat transfer efficiency.