Partial substitution of pulverized coal (PCI) by renewable biocarbon for blast furnace injection is a practical approach to reduce GHGs in the steel production sector. For using biocarbon as feedstock, it is critical to measure its grindability to understand its milling behavior compared to that of typical PCI coal, and thus help decide on adjusting grinding process of existing equipment or selecting appropriate milling equipment. From the significant differences in chemical and physical properties of biocarbon and PCI coal, grinding capacity variations are also expected. In addition, as the pyrolysis process modifies the chemical and physical properties of biomass, this could affect the efficiency of co-milling with coal and therefore co-injection in blast furnace ironmaking. The increase in milling capacity in turn would enable a reduction in the amount of coal used without the requirement for supplementary grinding facility. This paper presents an experimental investigation of the co-grinding behavior of coal and biocarbon produced by pyrolysis of woody biomass from two different conditions and post-processes. The main aim of this study was to identify or develop a method of grindability determination for coal, biocarbon blend, which would allow direct comparison between coal, biocarbon, and coal-biocarbon blends in terms of their grinding capacity. The study also includes the assessment of the possibility of adapting the existing coal and biomass grindability determination method for the evaluation of coalbiocarbon co-milling. The laboratory tests used focused on the influence of initial sample fixed mass and volume, composition of coal and biocarbon, the pyrolysis severity of biocarbon, and the densification process of biocarbon. The grinding behavior of biocarbon-coal mixtures determined by ASTM HGI procedure for coal is presented and compared in this paper. The findings show that milling tests conducted with fixed sample mass and fixed sample volume on same coal-biocarbon blend sample produce different and unrelated results. Moreover, grinding test results with fixed initial sample volume does not reflect material behavior in industrial grinding facilities. Therefore, it is suggested that the ASTM HGI method should be strictly followed to generate comparable grindability data for coal, biocarbon, and coal-biocarbon mixtures. [ABSTRACT FROM AUTHOR]