To reduce climate changes and to encourage usage of renewable energy resources are very important goals for EU. One of the resources, what is widely used at small power thermal energy sources, is biomass. Development of wood processing and incineration technologies and an increase of boiler’s automatization level, the more popular becomes impacted biomass fuel – pellets, briquettes. This paper is contributed to experimental research, which goal was to test pellet boiler and to determine pellet quality characterizing parameters. Results of the experiment were analyzed to determine pellet quality influence to boiler’s performance, especially taking into account NOx. Simple and two variable correlation functions and multi-parameter function (LINEST) was used for the analysis.