Although n-type Bi2Te3-based alloys are state-of-the-art thermoelectric material, their efficiency is still too low to satisfy its wide applications. Hence, it is imperative to improve the thermoelectric performance of n-type Bi2Te2.7Se0.3 (BTS). Here, we show that through a facile method of Sn addition in BTS a new SnxBi2Te2.7Se0.3 based nanocomposite embedded with in-situ formed SnBi and Te nanoinclusions ((SnBi + Te)/SnxBi2Te2.7Se0.3) is constructed, and its thermoelectric performance is enhanced substantially as compared to pristine BTS. Specifically, addition of 0.2 wt% of Sn in BTS causes 38% increase in power factor (PF) and 40% reduction in lattice thermal conductivity. The increased PF mainly comes from elevated Seebeck coefficient due to intensified energy dependent electron scattering caused by the interface potentials; while the reduced thermal conductivity originates from enhanced phonon scattering by the embedded nanoinclusions. Consequently, both high maximum figure of merit ZT (ZT(max) = 1.11 at similar to 370 K) and large average ZT (ZT(ave) = 1.03 at T = 300 K-500 K) are achieved for this sample, which are respectively 76% and 80% higher than those of BTS studied here.