In the last years magnesium alloys have been increasingly considered as attractive materials for the transportation industry. Extruded magnesium alloys have been found in the centre of interest combining their lightweight, surface quality with the wide range of possible achievable geometries. In the present study the alloy AZ31 has been chosen for investigation as one of the most common commercial magnesium wrought alloys. Round bars have been obtained through hot direct extrusion. After primary microstructural characterisation and mechanical testing in the as-extruded condition the specimens have been subjected to heat treatment consisting of different times (1, 2 and 4 hours) at different temperatures (200°C, 300°C, 400°C and 500°C) followed by cooling in air. Subsequently the specimens have been subjected to microstructural characterisation using light optical microscopy. Average grain size and grain size distribution have been determined using dedicated software. The microstructural analysis has been supported by microhardness testing on selected specimens. Additionally, the specimens have been subjected to tensile tests at room temperature applying a deformation rate of 1.3 × 10^-4 s^-1 as used for the material in as-extruded conditions. The obtained results have shown that heat treatment has no substantial influence on the microstructure characteristics of AZ31 up to 400°C. However, specimens treated at 500°C for one hour revealed a course grain, homogeneous structure with a substantial increase in grain size from 8 μm in as-extruded condition to 18 μm. This change in microstructure slightly reduced the strain-hardening exponent from 0.2 to 0.16. The remaining mechanical properties did not vary extensively when compared to the untreated, as-extruded material. It is assumed that the average grain size and grain size distribution have been influenced by complex thermomechanical treatments, which occurred during extrusion process as well as during heat treatment after heating at 500°C. By controlling the condition of heat treatment it is possible to improve the homogeneity of the material.