Abstract: The eigenvalues of a strongly regular graph with parameters(n,k,a,c),are k and θ,T which are the two roots of the quadratic equation x2-(a-c)x-(k-c)=0.The multiplicities mθ and mT can be determined from the equations mθ+mT=n-1,k+mθθ+mTT=0.And this gives an alternative method of determining its spectra.The complete multipartite graph Km(r) is illustrated.

Key words: strongly regular graph； complete multipartite graph； spectra； imprimitive graph；