Vivaldi antenna is one of the micro strip broadband antennas that can be applied in wide application such as wireless communication, imaging and radar. Many Vivaldi antenna research has been published to get higher directivity and smaller size of element. Beside that many research develop antenna performance to get higher gain and small beam width in the array. Smaller antenna can influence mutual coupling performance if it organized in closely spacing between elements. This paper will present mutual coupling of two coplanar Vivaldi antenna and radiation pattern of coplanar Vivaldi antenna in E plane array. For antenna element that has width of element smaller than a half wavelength of its lowest frequency, it will get worse S21 or S12 performance. Our purposed give method to improve mutual coupling performance and radiation pattern by adding slit structure in outer side of the coplanar Vivaldi antenna. It can improve mutual coupling reduction of 7.12 dB from -9.1437 to - 16.283 at 2GHz. The design can reach mutual coupling of -19.636 at frequency 4 GHz. Our purposed can improve E-Field of antenna of 1.9 V/m and SLL 0.4 dB at 2 GHz. It gets 1.19 times of main lobe improvement at 3GHz.