The effect of nonmesogenic solute on the nematic-smectic-A (NA) transition is examined within the Landau phenomenological theory. When nonmesogenic solute are added to a mesogenic compound consisting of both nematic and smectic-A phases, the NA transition temperature is depressed and a two phase region is formed due to the presence of the impurities of the solute. Decreasing the nonmesogenic solute to the pure compound, the first order NA transition becomes second order at the tricritical point in Landau approximation. The effects of nonmesogenic solute on the Frank elastic constants are also discussed.