Heat evolution of stress-induced structural disorder, ΔHs(ε), of a Zr55Al10Ni5Cu30 bulk metallic glass (BMG) during constant ram-velocity deformation at the glass transition region (Tg= 680 K) was deduced from in-situ measurements of temperature change of the deforming sample. At the transition from the linear to nonlinear viscoelasticity, the behavior of viscosity change with strain, η(ε), is qualitatively consistent with the enthalpy evolution of the structural disordering, ΔHs(ε), but not with the temperature change, ΔT(ε). It is concluded that the initial softening deformation is due to the stress-induced structural disordering. The change in the nonlinearity, -log ñ ≡ −logη/ηN, is found to be proportional to the ΔHs and the slope of ΔHs(–logñ) can be estimated to ∼ 400 J/mol, where ηN is the Newtonian viscosity. On the other hand, the temperature raise, ΔT(ε), is pronouncedly delayed as compared with the η(ε) and ΔHs (ε) at the transition, but is determined by the product of stress and plastic strain-rate, σ·εp, and is nearly proportional to it at the steady-state. The slope of ΔT(σ·εp) can be estimated to 5.2×10-2 K mol/W.