In tnis study, the author examines various calculating methods for the strength of inclined fillet joint; considers some of its characteristics, and tries to obtain the experimental formula and theoretical calculating formula of its strength.Seven kinds (α=0, 15, 30, 45, 60, 75, 90°) of double strapped inclined fillet joints (plate thickness 25mm, strap thickness 12mm, one weld length 80mm) are tested.1. Test results widely differ from the calculated results according to the old theoretical and experimental formula, and the ratio of strengths, end fillet to side fillet, is considerably high. And it does not seem to be true that the strength at α≅56°is minimum according to the projection method. But the strength is lower slightly at α=75°than at α=90°(side fillet).2. The calculated results according to the following convenient formula which is made by compromising projection method with Vandeperre's method approximate relatively well to the Vandeperre's test results:α≤tan-1R0 : R=0.3√R20cos2α+sin2α+0.7R0cosαα≥tan-1R0 : R=0.3√R20cos2α+sin2α+0.7cos(90°-α)where R=Ratio of strength of inclined fillet joint to side fillet joint.R0=Ratio of strength of end fillet joint to side fillet joint.α=The angle between the longitudinal direction of fillet and the perpendicular direction to the load.3. In both experiment and theory, the strength of inclined fillet joint seems to approximate generally to the strength of end fillet joint in the range of small angle α and to the strength of side fillet joint near the angle α=90°. But, as compared with the maximum shearing stress theory, the ratio of strength of end fillet joint and side fillet joint is high considerably, and it is likely that the minimum strength tends to happen at the angle α≤90°in the case of tensile load.