By designing a greatly improved high time-resolution polarimeter, measurements and calculations, at various pumping levels, of the time variation of polarization of the beams of a Nd34-barium crown glass laser have been made at liquid nitrogen temperature. It was found that, as far as the time-variation behavior of polarization is concerned, operation of the laser at 77°K or roam temperature makes no qualitative difference. Under normal pumping conditions, the majority of spikes have degrees of polarzation greater than 50%. The average degree of polarization per spike over one flash generally tends to go down with increasing pumping energy. Unfortunately the variation of polarization with time is so complicated that no preferred relation between polarization and time can be concluded. The sources of polarization have been discussed in detail, and the experimental results interpreted. The polarization properties of the polarized part of each spike ---e.g. the direction of rotation and orientation and ellipticity of the polarization ellipse-- have also been investigated. In addition, the existences of the multimode emission and the coherence between Ex and Ey of the light waves of this laser were justified by this experiment.