Background:Doxorubicin was an anthracycline antibiotic used for anticancer agent. This anticancer agent was one of the first line anticancer therapies which had clinical activity in breast cancer. Doxorubicin was metabolized in the body into its main metabolite, doxorubicinol. Long term use of this anticancer agent could accumulate the metabolite and caused cardiomyopathy, or hearts failure, due to its cardiotoxicity. This cardiotoxicity effect depended on the amount of doxorubicin and doxorubicinol accumulated in the body. Objective: This study aimed to calculate the level of doxorubicin and doxorubicinol in the blood plasma of breast cancer patients. Materials and Methods: Participants of this study were 30 breast cancer patients who received doxorubicin in their therapy regiment. The samples were analyzed using Ultra High Performance Liquid Chromatography – Tandem Mass Spectrometry (UHPLC – MS/MS). Sample preparation of 250 mL plasma was performed by protein precipitation using methanol. The mobile phase consisted of acetate acid 0,1% (eluent A) and acetonitrile (eluent B) with gradient elution, and the flow rate of  0,15 mL/min. Results: This method was linear in the range of 1 – 100 ng/mL for doxorubicin and 0,5 – 50 ng/mL for doxorubicinol. Results showed that the measured concentration values of doxorubicin and doxorubicinol ranged between 12,54 – 620,01 ng/mL and 1,10 – 27,00ng/mL respectively.