The　breakdown　phenomenon　at　freeway　has　both　practical　and　theoretical　interests.　Many　researchers　have　done　great　efforts　on　it.　As　a　result,　the　stochastic　concept　for　freeway　capacity　was　defined　and　some　approaches　were　developed.　However,　due　to　the　fact　that　it　is　impossible　to　observe　the　value　of　the　capacity,　it　is　difficult　to　obtain　the　exact　traffic　capacity　distribution.　In　this　paper,　traffic　operational　degradation　concept　was　introduced　and　degradation　model　was　developed　to　describe　traffic　breakdown　process.　A　stochastic　degradation　parameter　is　introduced　to　explain　the　phenomena　of　traffic　breakdown　occurring　at　various　flow　rate　levels.　Traffic　breakdown　probability　and　traffic　capacity　distribution　were　derived　theoretically.　Based　on　the　surveyed　data,　the　experimental　model　was　obtained　by　using　the　bootstrap　method.　The　results　indicate　that　Weibull　distribution　function　can　be　used　to　describe　traffic　capacity　stochastic　nature.