Abstract: Based on classification of shell ornaments and shapes of ostracods from the Qingshankou Formation to the 1st Member of the Nenjiang Formation in Songliao Basin, the authors studied the possible living environment of ostracods and variation characteristics of 12 types of ostracoda shell ornaments and 9 types of ostracoda shell shapes in the above strata. Studies show that, in the Late Cretaceous in Songliao Basin, ostracods with spinalis ornament were adapted to a deep or semi-deep lake; the species with nubble ornament preferred to live in clean, high pH value and high mineralized semi-deep or deep lakes, the species with deep alveoli, ornamental edge, long nubble or hunch were concentrated in deep or semi- deep lakes, and those with shallow alveoli, reticulation, veins, narrow hunch lived in the shore or shallow lake. It has also been found that the species with a triangular or rectangular shell were abundant in a deep or semi-deep lake; those with low trapezia or pentagonal shells lived in a deep, semi-deep or shallow lake, those with high trapezia shells lived in shallow water, those with ovoid shells lived in a shallow or shore lake, and those with reniform shells lived in a semi-deep or shallow lake. The ostracods in the 1st Member of Qingshankou Formation are dominated by individuals with ornamental edge and pentagon shells. Although shell ornaments and shapes of ostracods are varied in the 2nd and 3rd Members of Qingshankou Formation, they keep a corresponding relationship with the change of depositional environment in the lake. The ostracods in the Yaojia Formation are dominated by individuals with shallow alveoli and vein ornaments and ovoid shells, but a few individuals with deep alveoli ornaments and triangular and rectangular shells appear in the upper Yaojia Formation with the increasing depth of the lake. Although very few types of ostracoda ornaments are known in the 1st Member of Nenjiang Formation and dominated by deep and shallow alveoli ornaments, comparatively varied types of shell shapes are present in this member, such as ovoid, long reniform, rectangular, triangular and trapezia shells.