Recently, cannabidiol (CBD), one of major component of Cannabis Species, has focused on applications of Cannabis industry due to its various pharmacological effects. Interestingly, CBD can be converted into several psychoactive cannabinoids such as 9-tetrahydrocannabinol (Δ9-THC) and its structural isomers under acidic reaction conditions. In this study, chemical transformation of CBD in ethanol solution was conducted with variation of pH 2.0, 3.5, and 5.0 by addition of 0.1 M hydrochloric acid (HCl). These resulting solutions were derivatized with trimethylsilyl (TMS) reagent and analyzed by GC/MS-scan mode. Time profiles of CBD degradation and formation of transformed products were examined according to variation of pH and temperature. For identification of some products without authentic standards, their EI-mass spectra of cannabinoid-OTMS derivatives were interpreted according to their structural classes, suggesting mass fragmentation pathways. From time profile data, the acidity of the reaction solution was found to be an important factor in degradation of CBD. Degradation of CBD and formation of THC accelerated by applying low pH, increased temperature, and long process time.  Based on profile data and identified transformed products, their formation pathways from degradation of CBD under acidic reaction conditions are suggested. Among transformed products, seven components are known to have suspected psychoactive effects. Thus, industrial process of CBD in manufacturing food and cosmetic products should be carefully controlled. These results will provide important guidelines on the control of manufacture process, storage, fermentation process, and new regulation in industrial applications of CBD.