The structural change in the necking part of polymer monofilament has been investigated by means of the micro-photoelasticity. Its change depends on the humidity during storage of samples. Two samples were used in the present study, one of them was kept in moist atmosphere for a month and the other sample which was a part of the former was subsequently kept in a desiccator with P2O5 for a week. The wet sample is elongated by low load without yielding, and the dried sample makes yielding and necking under loading. In an unelongate portion of the necking part, the surface of monofilament is elongated more than its inner part. In the dried sample, it seems that the surface is harder than the inner and monofilament is easily elongated by concentrated force. After full elongation, the strains are same both in the surface and inner parts. And the behavior of elongation is different between under loading and stress relaxation.
