Pyroelectric and dielectric properties of Pb0.9La0.1TiO3 ceramics with grain sizes in the range of 1.3–6.0 μm were investigated. The temperature stability of the pyroelectric coefficient at room temperature was improved as the grain size became smaller. The pyroelectric coefficient of Pb0.9La0.1TiO3 at room temperature reached the lowest value near the grain size of 2.7 μm. Above 2.7 μm, the relationship between the pyroelectric coefficient and the grain size could be explained by the increase of 90° domain switching. However, below 2.7 μm, the relationship was not clear. The variation of the relative dielectric constant of Pb0.9La0.1TiO3 with the grain size showed a similar relationship with that of the pyroelectric coefficient. The variation of the pyroelectric figure of merit on voltage responsivity as a function of the grain size was similar to that of the pyroelectric coefficient because the pyroelectric coefficient varied more strongly than the relative dielectric constant.