In China, government commenced a compulsory program for the vaccination of all poultry in September 2005. We construct a simple mathematical model about the vaccination strategy. Our mathematical analysis, interestingly, reveals a situation that an increase of vaccination rate can lead to an increase of the number of total infected birds. This implies that the vaccination strategy which is aimed at preventing the disease spread assists the disease spread. In this paper, we investigate the "paradox of vaccination strategy" and obtain conditions of the paradox.