The transformation of amorphous Si layers to polycrystalline material induced byQ‐switched ruby laser single pulses of 20 and 50 nsec duration has been investigated. The analysis has been performed by transmission electron microscopy and by channeling measurements using 2.0‐MeV He+Rutherford backscattering. The average grain size of the polycrystalline layers increases with the incident energy density of the laser pulse in the range 0.6–1.7 J/cm2. A transition to single‐crystal layers is found for incident energy densities around 2.0 J/cm2. The grain size correlates with incident energy density (J/cm2) rather than incident power density (MW/cm2) for these pulse durations.