Under complex marine conditions, the critical heat flux (CHF) in reactor fuel assemblies differs from that in land-based reactors, necessitating further investigation. Given the current lack of clear understanding of the impact of even static inclined conditions on CHF, this study conducts CHF experiments in a circular tube under inclined conditions using R134a as the working fluid. The test section is a circular tube with an inner diameter of 8 mm and an effective heating length of 1600 mm. The experimental pressure range is from 1.6 to 2.7 MPa, the mass flux ranges from 1000 to 3000 kg/m2/s, and the inclination angle varies from 0° to 25°. The experimental conditions, once modeled, cover the pressure and flow range of pressurized water reactors. The study found that under low quality conditions at the outlet, inclination generally deteriorates CHF in circular tubes. As the inclination angle increases, this effect becomes more pronounced. In contrast, at higher quality, CHF remains unaffected by inclination. Under low void fraction conditions, the influence of inclination becomes more significant as pressure and flow velocity decrease. By integrating the Weisman bubble crowding model, the study elucidates the mechanism by which inclination affects CHF in circular tubes, taking into account the effects of inclination on single-phase heat transfer, turbulence intensity, and bubble motion and distribution. The varying impacts of these factors result in a non-monotonic influence on CHF. This research contributes to a deeper understanding of how marine conditions affect CHF and provides a reference for developing CHF mechanistic models suitable for marine environments.