Reconfigurable intelligent surface (RIS) emerges as a promising technology for the next generation networks. In this paper, we utilize the tools from stochastic geometry to study the performance of a RIS-assisted millimeter wave (mmWave) cellular network. Specifically, the locations of the base stations (BSs) and the midpoints of the blockages are modeled as two independent Poisson point processes (PPP) where the blockages are modeled by line boolean model and a fraction of the blockages are coated with RISs. The distinguish characteristics of mmWave communications, i.e., the directional beamforming and different path loss laws for line-of-sight (LOS) and non-line-of-sight (NLOS), are incorporated into the analysis. We derive the expressions of the coverage probability and the area spectral efficiency. The coverage probability under the special case where the blockage parameter is sufficiently small is also derived. Numerical results demonstrate that better coverage performance and higher energy efficiency can be achieved by the large-scale deployment of RISs. In addition, the tradeoff between the BS and RIS densities is investigated and the results show that the RISs are excellent supplementary for the traditional networks to improve the coverage probability with limited power consumption.