The improved device performance of organic light emitting diodes(OLEDs) and hole-blocking effect of SiO layer are presented. The interface between the organic layer and metallic anode of an OLED is crucial to the stability and performance of the device. A uniform and thin film of the alpha -septithiophene(alpha -7T) has been used as a buffer layer at the interface between the ITO electrode and hole transport layer(HTL). The insertion of a-7T layer has decreased the operating voltage and improved the external power efficiency. The Maximum EL intensity was over 17000cd/m(2) and the maximum external power efficiency at 2000cd/m(2) is 6.41m/W and that at 100cd/m(2) is 9.341m/W. Using the SiO layered in front of HTL as a hole-blocking layer, the sensitivity of the device efficiency on current density is investigated.