[en] [Background] Micro target X-ray source has attracted much attention in X-ray microscopy imaging. The focus spot size of X-ray directly affects the spatial resolution of sample. [Purpose] This study aims to investigate the influence of target materials and target microstructures on the emission spot of X-ray tube by simulation. The influence of different target materials on the focus size of X-ray tube is also discussed. [Methods] Monte Carlo simulation code MCNP5 was employed to calculate the focus size with five different target materials, i.e., Mn, Mo, Silver, Rt, Wu and three different target microstructures, i.e., hemisphere, cone and cylinder. The full width at half maximum (FWHM) of the light intensity distribution line was used as the standard to measure the spot size. [Results] The results show that the minimum focus size can be achieved by micro cone structure target at the incident electron energy of 100 keV. Moreover, different target materials have obvious influence on the focus size behind the target. The focus size decreases with the increase of the atomic number of the target material at the beginning, but expands to a certain extent with the further increase of the atomic number, showing a suppression trend. The influence of different target microstructures on the focus size is obviously related to the target type. The difference of focus size between cylinder and cone is up to 30 μm. Compared with the cylindrical tungsten target, the spot size decreases by 17.93% by using micro cone target structure and the silver target. [Conclusions] Compared with the traditional transmission X-ray tube, the micro target X-ray tube is easy to implement because it does not need strong electron focusing equipment. The methodology and the Monte Carlo simulation results are valuable for guiding the design and optimization of micro target X-ray tube source. (authors)