We employed pseudo potential and full potential methods to study the structural, electronic and magnetic properties of bulk MnSe by using two software: WIEN2k and PWscf. MnSe has 3 structural phases : NaCl , Zinc-Blende and hexagonal(NiAs). We considered both ferromagnetic and antiferroma gnetic phases in our calculations. Experimental results indicate that NaCl phase with type II magnetic ordering (NaCl (111)) is the most stable phase while our calculation with PWscf and GGA exchange-correlation energy showed the hexagonal phase has the lowest energy. Then we applied WIEN2k and different exchange-correlation energies (GGA, LSDA and LDA+U) to our calculation to solve this inconsistency. we got the best result within LDA+U method, confirming the stability of MnSe in NaCl structure and type II antiferromagnetic state. The calculated energy gap within LDA+U (~2eV) is also very close to the experimental result. Therefore LDA+U seems a reliable method for studying structural and electronic properties of MnSe. After bulk calculations, we studied structural properties of square cross sectional MnSe nanowire by using PWscf software and applying PBE-GGA exchange-correlation functional. We calculated the first 4 diameters of this nanowire and after full atomic relaxation obtained their cohesive energies. These calculations showed that antiferromagnetic is the stable hase for all 4 diameters. The Young's modulus of the first and second diameter nanowires was calculated and found to be larger than bulk value. We used two phenomenological model (macroscopic and microscopic models) to study and understand the behavior of the cohesive energies. In the first model the concept of dangling bond energies was used to analyze the nanowire cohesive energies while in the second model surface and edge energy parameters were introduced for macroscopic energy description of MnSe nanowires.