Steel beams lose much of their strength due to the temperature increase when they are exposed to fire. When their strength in any part of structure drops below the level the construction can withstand, they fail. There are a number of factors affecting the fire resistance of flexural steel beams. The most important ones are cross section, load size, load distribution and fire protection. This diploma thesis focuses on the fire resistance of steel beams, its determination by manual calculations and computer simulations using a three-dimensional finite element model. The result of the thesis is to monitor the response of steel beams exposed to fire under different load conditions and their relative comparison in terms of size of fire resistance.