RCaller is a software library which is developed to simplify calling R from Java. Despite it is not the most efficient way of calling R codes from Java, it is very simple to use and its learning curve is steep. It successfully simplifies and wraps type conversations and makes variables in each languages accessible between platforms. With the calling sequential commands facility, the performance is not lost through a single external process. Although R is single-threaded, multiple R processes can be created and handled by multiple RCaller instances in Java. A Servlet based application can instantiate many RCaller objects as well as it can use a single object by using sequential command invocation ability. The former use multiple environments which do not share the same variable pool, whereas, the latter shares a mutual variable pool and clients can communicate as well. RCaller is written purely in Java and it does not depend on any external libraries, that is, it is ready to run in any machines that Java and R installed. Simulation studies show that the other libraries such as Rserve and rJava outperform the RCaller by means of interaction times. As a result of this, RCaller is not suitable for the projects which have many clients that request relatively single and small computations. [RCaller: A Software Library for Calling R from Java - M. Hakan Satman]