Reversible crystal state structural phase transitions molecular materials without metal have been received increasing interest recently. Here we successfully synthesized a novel ternary neutral co-crystallized organic adduct hexamethylenetetramine.1-adamantane carboxylic acid.chloroform, 1, which accompanies with an interesting reversible phase transition close to the zero temperature (263 K). We confirmed this reversible phase transition by its switchable thermal dielectric behaviors and unit cell constants anomalies around the phase transition temperature (T-c). X-ray single crystal diffraction analysis above (1-RTP) and below (1-LTP) T-c suggests that the order-disorder motion from the adamantane ring in 1-adamantane carboxylic acid triggered the reversible phase transition.