To reveal the affecting mechanism of cooling rate on lamellar α precipitation, the precipitation behaviors of lamellar α phase in IMI834 titanium alloy during isothermal and non-isothermal heat treatments were quantitatively characterized using experimental analysis. Critical precipitation temperatures at various cooling rates were obtained using thermal dilatation testing. Using metallographic microscopy, electron microprobe analysis, and data fitting methods, the quantitative evolution models of average width, volume fraction, and solute concentration in the α and β phases were built for different temperatures or cooling rates. A comparison between the two precipitation behaviors showed that the average width and volume fraction of lamellar α phase under non-isothermal conditions were smaller than those under isothermal conditions. With increasing cooling rate, the average width and volume fraction were decreased significantly, and the critical precipitation temperatures were reduced. This phenomenon is mainly attributed to the decreased diffusion velocity of solutes Al, Mo, and Nb with increasing cooling rate.