We have previously found that the carbonization of coal under the load oi mechanical pressure was effective for producing metallurgical coke from slightly coking coals. In this paper, the effect of pressure load on the carbonization behavior of coal was investigated by measuring gas formation rate and several solid properties such as yield, amount of tetrahydrofuran (THF) soluble and pore volume distribution to clarify the mechanism by which the coking properties increase. By increasing the load up to 40MPa, the formations of H-2 and CO were significantly suppressed, whereas the solid yield and the amount of THF soluble increased. Furthermore, the solid yield was found to decrease significantly for a slightly coking coal when the load was released at 350 similar to 400 degrees C. This shows that a fairly large amount of volatile matters are retained in the coal under the pressure load. Judging from these results, thermal plasticity of coal are supposed to be enhanced because relatively low molecule compounds are retained in the coal due to effective hydrogen transfer under the pressure load. Analyses of the THF soluble by GPC and by H-1-NMR clarified that organic compounds with 2 similar to 3 aromatic rings contribute to the enhancement of thermal plasticity of coal.