GHG emissions produced by combustion of natural gas in district heating plants, electricity plants, co-generation plants or for domestic cooking does not represent GHG emissions from the whole life cycle of natural gas. The life cycle of natural gas includes pre-production, extraction, gathering, processing, transmission, storage, distribution – these steps produce significant CO2 and CH4 emissions. Co-firing biomass and natural gas is a step to decrease the rate of fossil fuel use but biomass size reduction for co-firing is an energy intensive process. Using data from published researches on energy consumption for woody biomass size reduction possible GHG emissions in the production of 0.5 mm and 0.1 mm woody biomass particles were calculated to investigate if fossil fuel use in the process of woody biomass fuel preparation for co-firing with natural gas results in overall GHG emission savings. Latvia’s grid electricity fixed CO2 emission factor was used in the calculations determining GHG emissions from electricity use. Results show that GHG emissions from woody biomass preparation in both chosen sizes are smaller than GHG emissions from the amount of natural gas that woody biomass particles would substitute however knowing the actual CO2 emissions from electricity would give more accurate results. Results also show that in accordance with Rittinger’s law achieving smaller sizes require increasing energy.