Orientation distribution function of glass fiber cylindrical particles suspended in laminar and turbulent shear flows in a water tunnel are experimentally measured by direct observations and measurements of the particles' spatial orientations and lengths. The noninvasive experimental technique employs a combination of a microscopic video-photography system and a computerized image analysis system. The orientation distribution function within a small control volume near the pipe's axis is determined by calculating the distributions of the frequencies of the particles Euler angles in 10 distinctive laminar and turbulent flow regimes. The orientation distribution function is found to be affected by the flow velocity gradients and the particles' rotational diffusivities, both acting in competition with each other. In the laminar flow regimes the longitudinal distributions are wide for small flow Reynolds numbers and become narrower, with a sharp maximum, with increasing Re. In the turbulent regime, broad distributions are found, with a tendency towards random distributions at large flow Reynolds numbers. It was found that in addition to the particles' rotational Péclet number, the flow regime has a profound influence on the particles' orientation. A dimensionless criterion is suggested and used to correlate the geometric mean of the longitudinal angular distribution with the flow parameters and the particles properties. The experimentally-measured orientation distribution functions are in qualitative agreement with the available theoretical results.