Abstract: The formation process of molecular clusters was considered in order to understand the boiling and nucleation of superheated liquid in microchannels and/or microstructure.The nucleation is inevitably accompanied with the volume increase of the clusters for the clusters to reach more stable state.This volume change causes pressure disturbance in the surrounding environment.Under microscale conditions,the pressure disturbance may play a leading role in phase change.The interaction of clusters with the pressure disturbance could cause the association and dissociation of clusters.The comparison between the propagation time of the pressure disturbance and the time of clusters growing to the critical radius in superheated liquid can determine whether the clusters are able to grow into boiling nuclei or vanish.Thus a critical microchannel size scale is obtained for "fictitious boiling" .The estimated value is in a satisfactory agreement with experimental data for water.