Using the Keck 10 m telescope data with the HIRES spectrograph, we analyzed the evolution of Lymanα forest clouds at z > 2 down to the HI column density 10 12.8cm -2. The number density per unit column density does not change with redshifts at lower HI cloumn density (NHI < 10 14cm-2), while the forest clouds at higher column density disappear rapidly. The cutoff b value, the thermal temperature indicator, increases as redshift decreases. The correlation strength seems to be stronger as redshift decreases.