Zirconium diboride ceramics were densified by hot pressing and spark plasma sintering with heating rates varying from 5 to 300℃/min. Slower heating rates produced larger grains due to the longer times spent at temperatures between 1500 and 1900℃, which is the temperature range in which ZrB2 grains coarsen. Heating rates above 50℃/min resulted in rapid densification, but this led to the retention of up to 3.3 vol.% of ZrO2 particles in the ceramics. After densification, changes to the microstructure were evaluated to interpret the effects of heating rate on thermal and mechanical properties. The flexure strength of ceramics processed by hot pressing up to 80℃/min was proportional to the inverse square root of the maximum grain size based on the Griffith criteria. Conversely, densification by spark plasma sintering, which had heating rates of up to 300℃/min, resulted in microcracks that decreased the elastic modulus from >500 GPa for pristine specimens to <485 GPa for microcracked materials. The use of heating rates >20℃/min also reduced the thermal conductivity due to the presence of retained ZrO2, but improved the strength by reducing the maximum grain size.