Packing is widely used for bypassing anti-malware systems, and the proportion of packed malware has been growing rapidly, making up over 80% of malware. Few studies on detecting packing algorithms have been conducted during last two decades. In this paper, we propose a method to classify packing algorithms of given packed executables. First, we convert entropy values of the packed executables loaded in memory into symbolic representations. Our proposed method uses SAX (Symbolic Aggregate Approximation) which is known to be good at large data conversion. Due to its advantage of simplifying complicated patterns, symbolic representation is commonly used in bio-informatics and data mining fields. Second, we classify the distribution of symbols using supervised learning classifications, i.e., Naive Bayes and Support Vector Machines. Results of our experiments with a collection of 466 programs and 15 packing algorithms demonstrated that our method can identify packing algorithms of given executables with a high accuracy of 94.2%, recall of 94.7% and precision of 92.7%. It has been confirmed that packing algorithms can be identified using entropy analysis, which is a measure of uncertainty of running executables, without a prior knowledge of the executable.