In this research work, the question of how important the particle size effect is in assessing radiological impact from a short-term radiological dispersal device incident is examined. A computer model, called puff particle size-dependent inhalation dose assessment (PIDA), was developed to support the task. The PIDA code is composed of submodels for atmospheric transport, dry deposition, resuspension, human exposure and dose analysis, with the particle size effect explicitly described in all of the submodels. The time-dependent nature of contaminant transport in the atmosphere during a short-term radiological incident was described by using a three-dimensional dispersion and one-dimensional advection Gaussian puff model. The results from the PIDA code were found to be in reasonable agreement with the experimental data from the Prairie Grass Project under various stability conditions and also with the calculation results from the CALPUFF code. The use of the PIDA code to examine the particle size effect in a short-term radiological incident showed that the particle size is one of the key parameters that contribute to the uncertainty of the estimated inhalation dose. The results also indicated that ignoring the particle size effect typically results in a conservative estimate of inhalation dose. In this regard, the use of an appropriately selected fixed value for particle size could be acceptable for a conservative estimate.