The longest relaxation times of double- and single-stranded lambda DNA (ds lambda-DNA, ss lambda-DNA) were studied by viscosity measurements in an oscillatory flow and by stress relaxation measurements. The results for ds lambda-DNA agreed well with chain retraction experiments of stretched single molecules; both were close to the relaxation time calculated from the Zimm model. The relaxation time of ss lambda-DNA is too small to be accurately measured by the stress relaxation experiments even in a very viscous solution. Such a small relaxation time makes it difficult to be studied in experiments of single molecule dynamics. For ds lambda-DNA in the intermediate time scale, the stress relaxes in a power law manner toward the long time scale with an exponent of -0.95, which is in good agreement with the Brownian dynamics simulations.