[en] Active carbon can decrease radon in local room because of its good capability with absorbing radon. The relations of the biggest decreasing radon efficiency with absorb coefficient, mass, volume of room, temperature, and humidity were studied. A theoretical model of decreasing radon efficiency by active carbon was established and verified with two sizes (4×8 mesh and 12×20 mesh) Philippine active carbon by the experiments. The result shows that the relative difference between theoretical value and measuring value is in ±4.7%. (authors)