Although it may be expected that, with increase in inter-nal pressure the bolt load in flanged joint would necessar-ily increase, this is not always true. This effect was test-ed experimentally by using strain gauges. Narrow gasket type was used with different materials and dimensions. This paper deals with the analysis of the obtained results as well as with the theoretical analysis. One of the most interesting effect is that the tensile load increases or decreases as the applied pressure increases. It can be seen that it depends upon gasket material and its dimensions. New shape for the screw (bolt) diagram has been proposed theoretically as well as prooved experimentally for the case when the bolt stress decreases with the increase of internal pressure.