This paper describes a new method to optimise roller-cone bit selection and operation by means of achieving the highest rate of penetration and bit life in a given formation that results in the lowest total cost for the hole. The performance in both areas can be maximised by controlling the weight applied and the rotary speed for the selected bit providing ideal hole cleaning. A Horizontal Drilling Rig (HDR) designed and used to perform full scale drilling tests in the laboratories of ITU Faculty of Mines. The block sample size of approximately 1.5 x 1.0 x 1.0 m was taken from KBI Murgul Mine and the geotechnical parameters of the samples were investigated. According to the values obtained, the appropriate roller-cone bits were initially selected from the product lists of four different manufacturers. The optimum operating parameters were sought for each bit on the HDR as rotational speeds and weight applied on bit were the controlled parameters and the penetration rate, torque, power consumption are recorded with the aid of an advanced data acquisition system. Finally, field tests were carried out to determine the bit life under the optimum operating conditions obtained in laboratory as this leads to the optimum bit selection and operation at KBI Murgul Copper Mine.