Seventeen circular hollow section (CHS)-to-plate T-joints were fatigue tested under constant amplitude stress range. The CHS-Plate T-joints are made up of cold-formed hollow sections of thicknesses less than 4mm. The load applied was “in-plane bending in the brace”. For each of the three connection series tested, stress concentration factors (SCFs) were determined in one of the specimen in each connection series. The results of the CHS-Plate T-joints from this investigation are compared to the fatigue test results and stress concentration factors of welded thin-walled (t<4mm) square hollow section (SHS)-to-plate T-joints from previous research. The results from the two types of connections are analysed to determine the effect of tube wall thickness on fatigue strength and the effect of SCFs on fatigue strength. Lower bound S-N curves for the thin CHS-Plate and SHS-Plate T-joints are determined through statistical analysis using the least squares method. Fatigue design curves for the welded thin-walled CHS-Plate and SHS-Plate T-joints are recommended based on a comparison between the statistically determined lower bound S-N curves from the test data and the existing family of design S-N curves from the Australian Standard AS4100 and Part 1.9 Fatigue of EC3.