The effect of fillet size and fusion penetration on statical strength of end fillet welded joint is experimentally investigated. Tensile tests are conducted using idealized model specimens of the joints made of a mild steel plate to prevent the metallurgical effect of welding. An empirical formula of breaking load per unit weld length is established as a function of leg length, effective fusion penetration, and strength of filler metal. The utility of the formula is confirmed by tensile tests of actual fillet welded joints. Reduction of the leg length due to the effective fusion penetration is also investigated in accordance with the empirical formula stated above, and the formula of reduction rate is developed as a function of the ratio of effective penetration to leg length.