The crystallization kinetics of fractionated polyethylenes under high pressure was studied by the DTA method for samples having molecular weights of 15500 and 150000. In extended-chain crystallization there was a saturation phenomenon on producing extended-chain crystals. The amount of saturation was dependent on the crystallization temperature. The saturation phenomenon seems to be an important factor for considering the mechanism of growth in extended-chain crystals. The Avrami exponent of extended-chain crystals was almost two. In the case of the folded-chain crystallization, the Avrami exponent was about three, irrespective of pressure. The crystallization rate of folded-chain crystals became more rapid as pressure was increased owing to the depression of the value of excess free energy of the fold surface. The crystallization rate of extended-chain crystals is ako considered to increase with pressure by indirect comparison, and seems to have a maximum crystallization rate near the melting temperature. The estimation of lamellar thickness is discussed for folded-chain crystals produced under high pressure.