Full understanding about how nanocrystals nucleate is mainly inhibited by two factors: (1) lack of a comprehensive theoretical framework which can explain and predict the occurrence of a nonclassical indirect nucleation pathway, and (2) lack of a high temporal and spatial resolution observation method to study the nucleation process. Here we introduce a single nanosecond laser pulse method with TEM observation to reveal the nucleation mechanism of hematite nanocrystals. The single pulse experimental design aims at revealing the phenomenon at the earliest stage of the crystallization process. The short nanosecond heating duration prevents the aging process which might obscure the precise nucleation information for the TEM study. By this method, it is shown that the nucleation pathway of hematite nanocrystals can be tuned by the energy input of the laser pulse. Hematite crystals, amorphous materials or both are nucleated depending on the energy of the first laser pulse. We believe that the short laser pulse method is a valuable method to study other nucleation phenomena at the nanoscale and will contribute to deepening our understanding about how different material structures form at the very early stage.