Higher figure of merit (ZT) in a SnSe based thermoelectric material is achieved by reducing thermal conductivity (κ) while keeping Seebeck coefficient (S) through a substitution of an isoelectric atoms, exfoliation from a bulk ingot, and a transformation process for porous structure. SnSe_1-xS_x nanosheets (NSs) are prepared by a hydrothermal Li-inter-calation and an exfoliation process from bulk ingots. The substitution of S atoms in SnSe and the fabrication of their SnSeS NSs contribute to the phonon scattering at a number of atomic disorders and nano-sized boundaries, leading to the effectively reduced κ value and finally the improvement of ZT. The introduction of the porous structure through the chemical transformation process results in the additional reduction of κ, which leads to the higher ZT. The fabricated porous SnSe_0.8S_0.2 NS show a maximum ZT value of 0.13 at 300K for the porous SnSe_0.8S_0.2 NS, which is ~27% and ~18% larger than the pristine SnSe and SnSe_0.8S_0.2 NS.