An effect of the distribution of vacant pores in RuO2 heating resistor elements on the printing characteristics of a thick film thermal printing head was examined. The head had an RuO2-glass heating resistor element which had some vacant pores attributed to gas evolution during the manufacturing process. The distribution of the vacant pores in each heating element was observed and quantitatively analyzed by an image processing technique using photomicrographs of the elements in correlation with the printing efficiency. The vertical distribution of the pores in the heating resistor elements significantly affected the printing characteristic. A poor thermal conductivity of vacant pores prevented heat conduction to a heat-sensitive paper and to a substrate, resulting in lower and higher efficiency for thermal printing, respectively.