Ruby/Mathematica is a library that provides an interface to the MathLink(TM) library. This library has some methods that implement a protocol for sending and receiving Mathematica(TM) expressions. Using this library, a Ruby program runs with Mathematica kernel in the background as a computational engine. In this library, MathLink functions should be called as methods of Ruby objects. Therefore the Ruby names have the `ML' prefix stripped. Since Ruby can handle multiple return values, some methods fetching elements from the link return the values instead of passing results in reference parameters as Ruby's customs. The representation of the data passed between Ruby and Mathematica is straight forward. Some examples using this library are included in the distribution.