A novel method is proposed to manipulate beam by modulating light phase through a metallic film with arrayed nano-slits, which have constant depth but variant widths. The slits transport electro-magnetic energy in the form of surface plasmon polaritons (SPPs) in nanometric waveguides and provide desired phase retardations of beam manipulating with variant phase propagation constant. Numerical simulation of an illustrative lens design example is performed through finite-difference timedomain (FDTD) method and shows agreement with theory analysis result. In addition, extraordinary optical transmission of SPPs through subwavelength metallic slits is observed in the simulation and helps to improve elements ' energy using factor. (c) 2005 Optical Society of America.