The objective of the research was to determine and compare values of the heat of combustion of barley straw from cultivation at two levels of potassium fertilization. The research were conducted using a calorimeter pursuant to the applicable standard PN-EN ISO 9831:2005. Based on the obtained results it was determined that moisture of barley straw and a dose of potassium used at the cultivation influenced the value of the heat of combustion of barley straw. At the moisture of 3.2 and 11.7% straw of barley which was fertilized with potassium with the dose of 150 kg∙ha -1 was characterized by a higher value of the heat of combustion, in comparison to the the second variant of fertilization with this component, which amounted to respectively 18.98 and 18.02 MJ∙kg -1. In case of barley, which was fertilized with a half lower dose of potassium, the heat of combustion of its straw, at the moisture of 3.2% was 18.3 MJ∙kg -1 and 17.51MJ∙kg -1 when the straw was of 11.7% moisture.