Abstract The excessive thinning generated in hydroformed products using tubular blank is attributable to the contact friction force restraining the tube wall movement on the tool inner surface. Therefore, it is fundamental and significant to take tube diameter as large as possible prior to designing a promising loading path. In this research, the tube diameter effect on hydroformability was investigated for a member with complex section of vehicle bumper rail. With changing tube diameter, loading paths for minimizing thinning were identified. Four types of paths defined according to whether feeding was imposed or not and whether pressure was applied during tool closing or not were employed for examining the formability including thinning. Finally, as a tool to enhance the thickness uniformity, tool with slant split planes was designed and the predicted results of formability were described.