The microstructures and mechanical properties of the friction welded pure Ti/AISI 321 stainless steel have been investigated. From the Ti side from the interface to the Ti base metal, the sequence of recrystallized grain, elongated grain and many twin embedded grain structures were observed. The reaction layers were formed within 0.2 μm thickness at the interface under the conditions of relatively longer friction time (t₁) and lower upset pressure (P₂). These reaction layers formed at the central interface were identified as (Fe, Cr)₂Ti, FeTi₂ or Fe₂Ti₄O and β-Ti, while those of the peripheral interface were identified as FeCr (σ phase), (Fe, Cr)₂Ti, FeTi and β-Ti. The σ phase was restrictedly formed at the peripheral interface. Higher mechanical properties were acquired under higher upset pressure condition due to higher compressive force between bonded materials, smaller grain size and narrower thickness of reaction layer. Therefore, maximum ultimate tensile strength of these joints was approximately 420 MPa with the conditions of 400 MPa of P₂ and 0.5 s of t₁.