In this letter, a monolithic missile radome for application in frequency modulated continuous wave radar (FMCWR) at missile warhead is proposed with improved radiation performance. The radome is cone-shaped and two patch antenna subarrays for transmission and reception are placed off the central line of radome. To address the issues of radiation distortion caused by radome, an improved radome with variable thickness is proposed. Theoretical analysis is conducted to reveal the working principle, which demonstrates that proper condition of radome thickness can minimize the adverse effects of field reflection and phase delay on radiation patterns. By further optimizing the radome thickness through simulation, the ripple variation of main beam is mitigated and radiation patterns become more symmetric. Simulated and measured results show that the radiation performance of the antenna with proposed radome has been significantly improved as compared with that of the antenna with uniform thickness radome (UTR).