Through the use of polyethylenes with different crystallinities as matrices, the effects of the matrix crystallinity on the percolation threshold and dielectric behavior of percolative composites have been investigated. The results suggest that the percolation threshold is negatively related to the matrix crystallinity, whereas the enhancement of the dielectric constant is positively related to the matrix crystallinity. A two-dimensional diagram is proposed to illustrate such relationships. In addition, it has been found that the insulator-conductor transition is much flatter in low-crystallinity-matrix-based composites, and this may be favorable for preparing threshold composites with a high dielectric constant and a low loss tangent.