PKC-beta inhibitor Ruboxistaurin (RBX or LY333531) can be used to reverse diabetic microvascular complication. However, it has not been previously established whether RBX can protect against ischemia/reperfusion (I/R) injury of cardiac microvessels in diabetic rats. STZ-induced diabetic rats were randomized into four groups and underwent I/R procedures. Cardiac barrier function and the region of cardiac microvascular lesion were examined. Cell monolayer barrier function was detected in cultured cardiac microvascular endothelial cells (CMECs) subjected to simulated I/R (SI/R). PKC-beta siRNA was transfected into CMECs to silence PKC-beta. Apoptosis Index of CMECs was detected by TUNEL assay and phosphor-LIMK2 protein expression was examined by Western blot analysis. RBX and insulin administration significantly reduced the cardiac microvascular lesion region and Apoptosis Index of endothelial cells (all P < 0.05 vs. no-treatment group). RBX decreased phosphor-LIMK2 expression (P < 0.05 vs. no-treatment group). RBX pretreatment and transfection with PKC-beta siRNA induced a rapid barrier enhancement in CMECs monolayer as detected by increased transendothelial electrical resistance (TER) and decreased FITC-dextran clearance (all P < 0.05 vs. no-treatment group). Meanwhile, RBX pretreatment and transfection with PKC-beta siRNA significantly decreased TUNEL positive CMECs and phosphor-LIMK2 expression in cultured CMECs (all P < 0.05 vs. no-treatment group). RBX pretreatment reduced F-actin/G-actin in cultured CMECs, reproducing the same effect as PKC-beta siRNA. These data indicate that PKC-beta inhibitor (RBX) may be helpful in attenuating the risk of severe cardiac microvascular I/R injury in diabetic rats partly due to its maintenance of endothelial barrier function and anti-apoptotic effect.