Diamond holds the highest figure‐of‐merits among all the known semiconductors for next‐generation electronic devices far beyond the performance of conventional semiconductor silicon. To realize diamond integrated circuits, both n‐ and p‐channel conductivity are required for the development of diamond complementary metal‐oxide‐semiconductor (CMOS) devices, as those established for semiconductor silicon. However, diamond CMOS has never been achieved due to the challenge in n‐type channel MOS field‐effect transistors (MOSFETs). Here, electronic‐grade phosphorus‐doped n‐type diamond epilayer with an atomically flat surface based on step‐flow nucleation mode is fabricated. Consequently, n‐channel diamond MOSFETs are demonstrated. The n‐type diamond MOSFETs exhibit a high field‐effect mobility around 150 cm2 V−1 s−1 at 573 K, which is the highest among all the n‐channel MOSFETs based on wide‐bandgap semiconductors. This work enables the development of energy‐efficient and high‐reliability CMOS integrated circuits for high‐power electronics, integrated spintronics, and extreme sensors under harsh environments.