An analytical solution is presented to describe the plasmon resonant wavelength of nanoparticles with arbitrary shapes. Calculation and analysis results demonstrate that the peak wavelength of the extinction spectrum for a nanoparticle of a certain material is determined by the shape parameter L only. The shape parameter L is in good fitting with a quadratic function of aspect ratio η, and its quadratic term coefficient a is a parameter that describes the acutance of a nanoparticle: the sharper nanoparticle is, the larger a is, and as a result, the longer the peak wavelength will be. This investigation can be used as a guideline in designing nanostructures for certain applications.