The effect of uncertainty in thermal diffusivity on thermal process lethality was investigated for a model food of 8 percent canned bentonite suspension in two different can sizes. A developed computer program that account for can headspace thermal resistance and finite thermal resistance during cooling was used in this investigation. Six levels of errors in thermal diffusivity (±2%, ±5% and ±10%) were used to generate computer simulation time-temperature data at the geometric centre for both cans. The calculated lethality values from the time-temperature data when errors were imposed on thermal diffusivity were compared to that of the standard process. The results of the investigation revealed that error in thermal diffusivity of (-2%, -5% and -10%) resulted in under-processing, while error of (+2%, +5% and +10%) resulted in over processing. The magnitude of error in lethality from +10 percent errors in thermal diffusivity is substantial for both short can and long can investigated reaching a value of 35.4 percent for the short can and 44.5 percent for the tall can. The results from this study clearly indicate that thermal diffusivity could be a critical control point in thermal processing of foods which requires corrective action if there is deviation from the true value.