The dependence of denderitic patterns and solute segregation on solid diffusivit y Ds is studied using a phase-field model which incorporates concentr ation field for Ni-Cu binary alloy dendritic growth. The computed results indica te that with the decrease of solid diffusivity Ds, the smaller become s the thickness of the solute diffusion layer ahead of the interface, which is a dvantageous to the growth of the sidebranching, and the more developed the side- branches. The smaller the solid diffusivity Ds, the more acutely the solute concentration in solid phase fluctuates. With the increment of solid diff usivity Ds, the fluctuations of solute concentration in solid phase a re reduced accordingly. An increased level of solid diffusivity Ds re duces the severity of microsegregation.