In this letter, two slot arrays with high radiation efficiency are proposed based on the empty substrate integrated waveguide (ESIW) technology. The arrays consist of four FR-4 substrate layers that function as cavity loading, radiation slots, ESIW feeding network, and feeding port, respectively. The $1\times 6$ -slot array shows a measured −10 dB bandwidth of 12.7% (36.1–41.0 GHz) with a measured maximum gain of 15.5 dBi and radiation efficiency of 90.7% at 38 GHz. The $8\times 6$ -slot array shows a measured −10 dB bandwidth of 12.1% (36.4–41.1 GHz) with a measured maximum gain of 24.0 dBi and radiation efficiency of 86.5% at 39 GHz. Additionally, stable radiation patterns are obtained in the operating band. These arrays provide a low-cost way for broadband millimeter-wave wireless applications including the fifth-generation mobile communications.