Friction stir welding (FSW) was conducted on 0.5-in. thick Ti-6Al-4V plate.  Plates with embedded thermocouples were welded to obtain thermal profiles at various locations in stir and heat-affected zone during the FSW process.  The results indicate that the highest peak temperature in the stir zone is above beta-transus temperature.  However, the peak temperature at the location near backside in the stir zone is below the beta-transus temperature.  The resultant weld microstructure was characterized in an attempt to understand the thermal history.  Tensile properties of FSW were evaluated and compared to that of pulsed gas metal arc welds in the as-welded and postweld heat treated conditions.  Unique process characteristics in FSW Ti alloy will be discussed and compared with FSW steel.