The effect of addition of n-octylamine (OA) on structural transition in aqueous (D2O) micellar solutions of SDS (sodium dodecyl sulfate) was studied by viscosity and small-angle neutron scattering (SANS) at various temperatures. The distinct rise in relative viscosity of SDS solution on the addition of a certain OA concentration (similar to 0.03 M) has been attributed to change in micellar shape from a sphere to a rod, At higher OA concentration, these rods overlap giving rise to large increase in viscosity. SANS distribution showed a well-defined peak for 0.3 M SDS. With the OA addition (0.02-0.065 M) shift in the peak position toward low and-values was observed. This is due to increase in intermicellar distance. The SANS distribution of 0.3 M SDS + 0.065 M OA, studied as a function of temperature; showed that micellar size decreases on heating. The SANS spectra were analyzed using various existing models. The micelle dimension (bla) and mean aggregation number (ii) were computed over a range of OA concentrations and temperatures. The observed results are interpreted in terms of mixed micelle formation with concomitant micellar growth.