Using the systematic evolutionary structure searching method and the density functional theory, a new planar nanostructure composed of 5-6-8 rings referred as pho-B2N3 to stand for the penta-hexa-octa geometrical characters is proposed, which is dynamically stable due to the absence of imaginary mode in the calculated phonon spectrum. The thermal stability to withstand the temperature as high as 2000 K against melting and the mechanical stability to bear the strain as strong as 14.9% have also been proved. Furthermore, its band gap could be continuously tuned by strain between 0.38 and 2.13 eV, showing potential usage in field effect transistors. (C) 2016 Elsevier B.V. All rights reserved.