In recent years, measurement of residual stress by ultrasonic method is developing because of nondestructive nature, portable equipment and being cheap and fast. In this research, the Capability of ultrasonic method by using longitudinal critically refracted or Lcr wave in measurement of longitudinal welding residual stress has been scrutinized. For this purpose, two plates of aluminum alloy series 5000 were joined by TIG welding method. Measurement of longitudinal residual stress by ultrasonic method was done in closeness of surface via 5 MHz transducers based on acoustoelasticity theory. In order to create Lcr wave and transmit it into specimen, an ultrasonic wedge was made based on Snell’s law. Also a triaxial table was used to control the wedge movement and keep the pressure on it fixed. In order to calculate residual stress and increase in accuracy, acoustoelastic constant for each three welding zones, including weld metal, HAZ and base metal was obtained separately from uniaxle tension test. In order to validate ultrasonic method results, measured longitudinal residual stress by x-ray diffraction method in 5 points on the specimen surface was used. Finally after comparing the results of two used methods with each other, good agreement was seen which indicates the good ability of ultrasonic method in measurement of longitudinal residual stress.