The performance of bolt-nut connections can be improved by enhancing fatigue life of the connections. This can be accomplished by reducing the stress concentration in the threads of the connection. This investigation consists of two parts. In this part (part I), load distribution in threads of some ISO bolts is computed by three-dimensional numerical simulation and Stockley-proposed relations. The results show a close agreement between Stockley relations and the simulations for nearly all bolt sizes. The results indicate that stress concentration is nearly constant regardless of the bolt size. It is also found that the load percentage carried by the first thread varies from 35% for M6 and reaches to 58% for M20 and M30 ISO bolts. The results suggest that the rate of load distribution changes at a point of inflection, i.e. the rate after the inflection point diminishes as the bolt size decreases, whereas before this point, the trend of the rate is reversed. In part II (to be submitted separately), various techniques are employed for the reduction of stress concentration and enhancement of fatigue life of the connections. The techniques are evaluated by numerical simulations and fatigue tests.