The study covers the analysis of ultrasonic welded joints made of ABS polymer. The quality of the welds was verified by an ultrasonic test and a tensile test. The research aims to determine the quality and reliability of the data gained from the ultrasonic test and includes an analysis of the welds themselves. Tensile tests were done, through which we obtained data of the breaking forces. On the broken surfaces a visual assessment of the weld zone was done. The area of the welded surface was calculated and compared with the data of the tensile strength tests and ultrasonic tests of bonding area. The ultrasonic testing gave us data on the melted surface inside the weld. The results of the ultrasonic testing prove to be reliable, considering a certain amount of error and correct interpretation. We found that the data of the ultrasonic testing cannot be directly linked to the weld strength, as it also depends on other factors.