Specimens of carbon steel (CS) and stainless steel (SS) 316 was joined together by double sided butt joint Friction Stir Welding method. Effect of the tool rotational speed and specimen preheat temperature on resultant microstructure and mechanical behavior was studied. Constant traveling speed of 100 mm/min, tool rotational speed of 500 and 1000 rpm in combination with preheat temperature of 50 and 100 °C was employed for this study. No sign of root crack was observed in all specimens. Optical microscopy identified four distinct zones characterized as the stir zone, thermal mechanically affected zone (TMAZ) in both SS and CS specimen, and heat affected zone (HAZ) in CS specimen. No HAZ in SS was observed. Scanning electron microscopy revealed formation of a thin void layer on several locations at the boundary between the TMAZ and the re-oriented region of SS side, while on the CS side no voids were identified. The grain size on the TMAZ of CS was observed to be smaller compared to the HAZ region. No sign of root crack was observed in all specimens. The specimens were subjected to U bend. Specimens joined with 1000 rpm rotational speed developed a brittle behavior and failed in U bend test. The extent of the crack and brittleness was proportional to the applied preheat. U bent specimens from 500 rpm rotation demonstrated a good stress corrosion cracking behavior in 30,000 ppm solution of sodium chloride (NaCl). © 2018, The Indian Institute of Metals - IIM.