Damping　Reduction　Factor　plays　a　key　role　in　scientific　literature　and　Technical　Codes,　but　till　now　existing　formulations　present　differences　and　inconsistences　probably　because　obtained　by　integration　of　real　recorded　events,　thus　sensible　to　specific　used　data.　This　paper　investigates　the　relation　between　damping　reduction　factor　and　earthquake　duration　by　means　of　random　vibration　theory.　A　stochastic　process,　that　is　non-stationary　and　filtered,　is　used　to　model　a　seismic　event.　The　modulation　function　is　suitably　chosen　to　describe　earthquakes　characterized　by　different　durations.　The　stochastic　process　peak　theory　allows　to　calculate　damping　reduction　factor　after　the　definition　of　the　probabilistic　response　of　a　simple　linear　visco-elastic　oscillator.　The　variability　with　seismic　duration　for　different　soil　conditions　and　damping　ratios　is　investigated.　The　study　points　out　that　damping　reduction　factor　is　more　sensitive　to　seismic　duration　in　the　range　of　high　period　and　on　rigid　soil　with　respect　to　other　conditions.　The　results　show　that,　if　damping　ratio　or　effective　duration　values　are　increased,　the　damping　reduction　factor　value　diminishes.