The effective interaction between two infinite, uniformly charged planes immersed in electrolyte solution having only monovalent counterions is calculated using Poisson-Boltzmann equation. Meanwhile, the influence of the boundary (a infinite, uniform charged plane) on a charged colloid sphere is investigated using the Derjaguin approximation. It is found that there is only repulsive force between two charged planes in the absence of microion correlation. Furthermore, the variation in the boundary condition has little effect on their effective interaction. The conclusions are similar with that of the charged-plane and charged-colloid-sphere system.