133P/Elst-Pizarro is the first recognized main-belt comet, but we still know little about its nucleus. First we use mid-infrared data of Spitzer MIPS, Spitzer IRS, and the Wide-field Infrared Survey Explorer to estimate its effective diameter, , and mean Bond albedo . The albedo is used to compute 133P's temperature distribution, which shows significant seasonal variation, especially polar regions, ranging from ?40 to ?200 K. Based on current activity observations, the maximum water gas production rate is estimated to be , being far weaker than of JFC 67P at a similar heliocentric distance of ?2.7 au, indicating a thick dust mantle on the surface to lower down the gas production rate. The diameter of the sublimation area may be <?200 m according to our model prediction. We thus propose that 133P's activity is more likely to be caused by sublimation of a regional near-surface ice patch rather than a homogeneous buried ice layer. Such a small near-surface ice patch might be exposed by one impact event, before which 133P may be an extinct comet (or ice-rich asteroid) with an ice layer buried below ?40 m depth. The proposed ice patch may be located somewhere within latitude-50 ? 50° by comparing theoretical variation of sublimation temperature to the constraints from observations. The timescale to form such a thick dust mantle is estimated to be >100 Myr, indicating that 133P may be more likely to be a relatively old planetesimals or a member of an old family than a recently formed fragment of some young family.