This study investigated the main origin of the thermochromism of a novel compound, 1,3-bis(hydroxyethylamino)-4,6-dinitrobenzene (BDB). In order to unravel the relative dependence of reversible color change between yellow and orange on proton transfer reaction and crystal-crystal phase transformation, kinetic experiments were executed. The recovery rates of thermochromic transition, proton transfer reaction, and crystal-crystal phase transformation were calculated from the data measured through UV-vis spectrophotometry, IR spectrometry, and X-ray diffractometry with a lapse of time at fixed-temperatures (30, 40, and $50^{circ}C$) after cooling from heated temperature ($150^{circ}C$). The kinetic experiments revealed that the recovery takes place more quickly at lower temperature. At each fixed-temperature, the recovery rate is in the order of the crystal-crystal phase transformation > the proton transfer reaction > the thermochromic transition. It is concluded that the proton transfer reaction is the main origin for thermochromism of BDB in solid state and the crystal-crystal phase transformation is a collateral one.