Lifetime　calculation　based　on　crack　growth　model　and　the　strength　degradation　model　were　analyzed,　respectively.　A　mathematical　law　which　reveals　a　proportional　relationship　between　initial　crack　size　and　calculated　lifetime　was　found　and　was　demonstrated　theoretically　for　crack　growth　model　with　the　form　da/dt=f(K),　It　was　pointed　out　that　the　lifetime　of　solid　materials　is　a　process　of　strength　degradation　caused　by　not　only　crack　growth　but　also　many　factors　like　environmental　corrosion　and　creep　damage.　Failure　occurs　only　when　residual　strength　declines　to　the　level　of　applied　stress.　Experiments　of　static　fatigue　and　crack　growth　rate　were　performed　using　glass　sheet　specimens　under　uniaxial　tension.　The　comparison　between　the　experimental　results　and　calculated　lifetime　indicated　that　the　lifetime　estimated　by　strength　degradation　model　was　closer　to　real　results　than　that　predicted　by　the　crack　growth　model　for　different　crack　sizes　under　the　same　KIi.