In the W flat with Weigert effect, both dichroism and birefraction should be showed simultaneously, but the birefraction is neglected generally because of it′s small. In this paper, we give a experimental result showing a remarkable birefraction as the dichroism is very strong (the phase difference between extraordinary and ordinary light be able to 80° and the value of n e-n o～0.02). A series of data and curves of the experiment are obtained. A principle and experimental method for determining the dichroism and the birefraction of a W flat are suggestees.