<p>Creep tests were conducted on weld joints under welding conditions with high creep performance. 2 1/4Cr-1Mo steel, which is widely used in high-temperature components such as boilers and piping of power and petrochemical plants, was selected as the target material. The creep performance of weld joints is governed by the stress distribution in the fine-grained heat affected zone (HAZ) during welding, which in turn depends on the HAZ shape. In this study, we demonstrated that creep performance can be controlled by welding conditions. The welding conditions were determined using a tandem Bayesian model that linked welding conditions and creep life through HAZ shape factors. This framework is a probabilistic combination of two surrogate models: a Gaussian process that predicts the HAZ shape from welding parameters, and a Bayesian linear regression that estimates the rupture life. The search targets are the number of layers, layer thickness, and heat input. Long-life conditions that were 1.8 times longer than the maximum life of the initial training data were explored. Two other conditions were selected: standard welding conditions and short-life welding conditions. Welding and creep tests were performed, and the results of the creep tests were in the order of long &gt; standard &gt; short. The HAZ shapes reflected the differences in life, as expected from the welding conditions. In conclusion, it was proved that the creep life can be controlled by adjusting the welding conditions.</p>