Detailed　experiments　of　fracture　toughness　in　which　SENB　specimens　of　five　different　thicknesses　were　included　were　carried　out　to　investigate　the　size　effect　in　the　ductile　to　brittle　transition　temperature　region.　It　is　found　that　the　fracture　toughness　of　the　upper　shelf　increases　with　the　thickness　of　the　specimens　with　the　similar　geometry.　While　the　fracture　toughness　of　the　lower　shelf　decreases　with　the　thickness　in　the　range　of　4mm　to　12mm　and　then　drops　up　from　12mm　to　16mm　with　the　appearance　of　shear　lips　which　present　the　shearing　fracture　under　the　plane　stress　state.　The　tearing　modulus　dJ/da　which　determines　the　resistance　to　stable　crack　growth　increases　with　the　increment　of　thickness　and　the　reduction　of　the　temperature.　The　results　of　the　stress　triaxiality　increasing　with　the　reduction　of　the　thickness　explain　well　the　experimental　results.