Spherical fine particles with various diameters (70, 160, and 400 Å and 35 μ) were mixed with low-density polyethylene (LDPE). The oriented composites were made by necking drawing. Thus the oriented polymer composites were hexagonal symmetric. Their elastic properties were determined by five compliances or stiffness constants. Four of them, i.e., S33, S11, S13, and S44, were determined for the oriented composites filled with 70-Å and 35-μ fillers. All the compliances of the 70-Å filler composites decrease with filler content, whereas in those of the 35-μ filler composites this relation was reversed. The Young's moduli of the oriented composites filled with relatively small particles (70, 160, and 400 Å) in the restretching directions 0, 45, and 90° against the original draw axis increased with filler content and with decreasing filler size, whereas those of the 35-μ filler composites decreased with filler content. These results show that extremely small particles comparable to the size of the LDPE in the crystalline region exert a considerable reinforcing effect on the oriented polymer matrix.