This paper deals with the fatigue durability of typical welded gusset joints in steel bridges strengthened by externally bonded CF (carbon fiber) sheets using VaRTM (Vacuum assisted Resin Transfer Molding) technique. VaRTM is a composite fabricating technique, can be used to apply CF sheets on cracked steel structures. The strengthening operation work have been proven to be very efficient and convenient even on complex shapes of structures due to the flexibility of this method. Perfectly close contact particularly between weld beads and the CF sheets provides a great advantage on reduction of the high stress concentration which is usually occurred at the fillet weld toe at the end of gusset plate. The reduction of stress concentration at weld toe was analytically investigated, where the 3D FEA (finite element analysis) model was simulated from a number of image data taken from actual experimental specimens by digital camera. The target specimens of welded gusset plates were fabricated and subjected to cyclic load. The fatigue tests of two types of specimens, non-strengthening and strengthening specimens using VaRTM technique have been conducted. The strengthening specimens are prepared with 23 layers of CF sheets (high-strength type), equivalent to reduction of stress concentration of approximately 1/3. Furthermore, in order to prevent the debonding of adhesive at CF sheet end, taper is designed under condition of fatigue limit of adhesive using theoretical calculation. The strengthening effects have been evaluated under applied nominal stress ranges, converted nominal stress range and hot-spot stress range experimentally. The result shows that the fatigue durability of welded gusset joints strengthened by CF sheets using VaRTM technique can be remarkably improved, reaching fatigue limit (10 million cycles) and over JSSC-A grade.