  Starting from a gauge invariant treatment of perturbations an analytical
expression for the spectrum of long wavelength density perturbations in warm
inflation is derived. The adiabatic and entropy modes are exhibited explicitly.
As an application of the analytical results, we determined the observational
constraint for the dissipation term compatible with COBE observation of the
cosmic microwave radiation anisotropy for some specific models. In view of the
results the feasibility of warm inflation is discussed.
