Abstract: To evaluate the detection performance of colocated multiple-input multiple-output (MIMO) radar, this paper proposes an approximate derivation of colocated MIMO radar equivalent array gain. First, the signal-to-noise ratio (SNR) improvement factor of the colocated MIMO radar array output from coherent integration perspective was analyzed. Then, the array output's SNR expression was derived. Eventually, the colocated MIMO radar equivalent array gain could be achieved via comparing the array output's SNR expressions of colocated MIMO and SISO radar. Simulation shows that, if the number of array element is increased two times, the array gain of phased-array radar would be increased by 3.0 dB, while the colocated MIMO radar would just be increased by 1.5 dB. It seems that, in order to achieve the same detection performance as phased-array radar, the colocated MIMO radar could use long-time integration method to compensate the loss of equivalent array gain.