The effect of heat treatment at 700-1500°C on the mechanical strength, micro strength and pore structure of carbonaceous materials, including coke, char and coals, were studied using tensile test, ultra micro indentation and image analysis. Strength of chars and pyrolysed coals was strongly enhanced by heat treatment at temperature below 1100°C; strength of cokes was slightly degraded after heat treatment at 1500°C. Mechanical strength of carbonaceous materials was demonstrated to be significantly affected by micro strength and porosity. Micro strength of chars and coals was significantly enhanced by heat treatment, whereas micro strength of cokes was only marginally increased by heat treatment. The major growth in the micro strength of chars and coals took place at annealing temperature below 1100°C. Porosity of chars and coals significantly increased during annealing at temperatures below 1100°C. Further increasing annealing temperature from 1100-1500°C caused marginal porosity evolution in pyrolysed coals and chars. Porosity of cokes increased slightly in the temperature range of 1300-1500°C.