In this paper, a novel design of the metamaterialbased microstrip antenna with the ground slots is proposed for gain-enhanced dual-band operation. To realize the dual-band operation of the left-handed and the zeroth-order resonant mode, six equilateral triangular patches which are consists of the coupling gap and the through-hole via are isotropically arranged and the feeding structure is wired to one patch. The equilateral triangular ground slots are embedded for the gain improvements of the proposed antenna at each resonant mode. By only increasing the slot size ranged from 0 mm to 11 mm without changing the radiator elements, the antenna gains have been excellently increased at each operating mode. The measured gains of the proposed antenna are improved by up to 11.69 dB in the left-handed and 11.18 dB in zeroth-order resonant modes at the slot size of 11 mm, respectively in good agreements with the simulated results.