Testing of concurrent programs is much more difficult than that of sequential programs. A concurrent program behaves nondeterministically, that is, the program may produce different results with the same input data according to execution timings of the program. In testing of concurrent programs, test data must specify not only input data but also sequences of statements. Ordered Sequence Testing Criterion for length k (OSC k ), which was proposed by the authors, requires execution of all sequences of length k of concurrency statements which cause concurrent actions in a concurrent program. A monitoring tool has been developed for applying the testing criterion OSC k to the testing of C concurrent programs on UNIX system. The tool measures coverage with regard to k-Tuples of concurrency statements (OSC k ) in source codes of a C concurrent program using a probe insertion method. The analysis of the tool's output for a practical C concurrent program shows not only applicability of the tool for testing of concurrent program but also the necessity of a supporting tool for forcing execution of concurrency statements.