In recent years, wireless sensor network technology has been frequently used in various fields, thereby making it increasingly important to ensure that the data being transferred remains confidential. Symmetric-key cryptography, which does not require high computing capacity, is a method that is generally used because the computing and memory requirements of sensor nodes are low. An implementation of symmetric-key cryptography is needed in order to achieve high-speed processing of sensor nodes. In this paper, we propose and evaluate a fast implementation scheme of the Advanced Encryption Standard using an inline-assembler that is suitable for the ATmega1281 microcontroller. This microcontroller is a part of the Atmel AVR series and is often equipped with sensor nodes for use in wireless sensor networks.