Nowadays, Ease of accessibility to natural oil and gas wells has fallen significantly due to continues drilling operations. Therefore, oil fields came after deep drilling and horizontal and directional drilling methods. However, due to more complex conditions in these wells, rods fails mostly. This failure occurs in rod joints as the weakest link. In this research, API NC46 standard tool joint has been investigated considering real oil well condition. The goal is to find the weakest points in the drilling rod and present an economical and practical solution to increase the life time. Results show that the maximum stress occurs in the first engaged thread of pin and last engaged thread of box which makes these areas more vulnerable. In the following, using optimization of joint main parameters including threads geometry, make-up torque and shoulder sealing design, a new plan will represent that have higher fatigue life than the similar API standard tool joint. Maximum stress concentration factor of improved tool joints relatively decrease from 3.47 to 2.86 compared to standard joint. In addition Experimental tests show that the average fatigue life of improved joint is 2.3 times more than the standard tool joint.