This article presents the growth of thin oxides by rapid thermal processing (RTP) under different O-2 pressures, indicating that high pressure oxidation tends to make oxide thickness less uniform as oxidation time increases. However, oxide under low pressure oxidation tends to saturate in thickness and a "self-compensating" phenomenon occurs. Thus in order to grow ultrathin gate oxide in a desired thickness regime with good thickness uniformity, we developed a technology called high then low pressure oxidation. The experimental results show that in thickness and electrical properties the uniformity of oxide obtained by oxidation under high then low pressure would indeed be improved.