An apparatus measures residual stress in a sample under test by measuring the penetration of an indenter on a unprocessed sample under test, and after processing of the sample under test, measuring again the penetration of the indenter on the processed sample under test, and deriving from the two penetration measurements the residual stress in a sample under test. The apparatus and method of the preferred embodiments are especially useful in determining residual stress in a printed wiring board. In this manner a direct measurement of residual stress is possible without destroying the printed wiring board.