The SrTiO3 (STO) compound exhibits a variety of unique physical and chemical properties such as remarkable conducting state at interfaces in the SrTiO3/LaAlO3 (STO/LAO) system.[1] Recent studies found that inaccurate laser influences or deposition geometry during the pulsed laser deposition (PLD)process can result in nonstoichiometric STO[2-4] and LAO films.[5] However,nonstoichiometric material transfer existing in the fabrication process of STO possibly leads to the formation of Sr-or Ti-rich secondary phases with new physical properties.[2,6]Therefore,S+1TinO3n+1 and STin+1O3+2 (n=l,2,3),known as the Sr-rich Ruddlesden-Popper (RP)phase[7,8] and the Ti-rich Magnéli (M) phase,[9] have been researched in experimental[10-12] and theoretical works.[13-15] The RP and M phases are regarded as natural superlattices for important applications,e.g.,tunable high dielectric,thermoelectric properties and texture engineering.[16-18] Excess Ti or Sr atoms will cause lattice expansion of STO,[3,19] whereas the details of Sr/Ti ratio impacting on lattice expansion has not been reported.Moreover,during the synthesis of STO films,partial dislocations were often observed in Ti-rich STO phase,[10] but hardly emerge in Sr-rich cases.[3,12,20] It is known that dislocations play an important role in many applications,such as ferroelectric,dielectric,catalysis or their electronic properties.[21,22] Because structural modifications to the inteal parameters and changes in lattice parameters are to be expected with structures of increasing n,the question of how exactly these quantitles change with n can be well answered with firstprinciples calculations.[13] In this Letter,we focus on the influence of Sr/Ti ratio on the lattice expansion,dislocation behavior and structural stability of Sr-rich RP phase and Ti-rich M phase by using first principle method based on density functional theory.