In this paper we apply the concept of thermodynamic geometry to the Bañados-Teitelboim-Zanelli ͑BTZ͒ black hole. We find the thermodynamic curvature diverges at the extremal limit of the black hole, which means the extremal black hole is the critical point with the temperature zero. We also study the effective dimensionality of the underlying statistical model. Near the critical point, the picture is clear; the spatial dimension of the underlying statistical model is just one, which agrees with other results. However, far from the critical point, the dimension becomes less than one and even negative. In order to interpret this result, we resort to a qualitative analogy with the Takahashi gas model. ͓S0556-2821͑99͒04516-6͔ PACS number͑s͒: 04.70.Dy, 04.60. Kz, 05.70.Jk Over the decades the statistical interpretation of blackhole entropy has been one of the most fascinating subjects. There have been many approaches to the problem, although nothing was completely successful. One curiosity about black-hole thermodynamics is that it looks different from an ordinary thermodynamical system, due to the negative heat capacity. This makes it hard to compose its thermodynamic ensemble and to make the underlying statistical model. One way to study the statistical aspects is to assume the microcanonical ensemble for an isolated black hole. Actually, in this way one can understand the critical behavior for the near extremal black hole. The critical exponents satisfying the scaling law even tells us the dimensionality of the underlying statistical model.In this paper, we suggest another tool useful for studying the statistical properties including the fluctuation, the critical behavior, and so on. This is the thermodynamic geometry. It defines a metric on the space of thermodynamic variables. ͑Of course, it has nothing to do with the geometry of space and time.͒ Here, the thermodynamic potential becomes the geometrical potential generating the metric components. The thermodynamic variables constitute the coordinates for the geometry. Details will be given below through the example of the Bañados-Teitelboim-Zanelli ͑BTZ͒ black hole ͓2͔. As is known, the BTZ black hole could play an important role in understanding entropy and some dynamical properties of certain five-and four-dimensional black holes in supergravity theories, because of the U duality between the BTZ black hole and those high-dimensional black holes ͓3͔. For a review see ͓4͔.In general, it is technically difficult to define the zerotemperature critical point, as is the case with the black hole. Conventional definition for the zero-temperature critical point is the point where at least one of the second derivatives of some thermodynamic potential diverges ͓1͔. However, in the language of thermodynamic geometry one can define it unambiguously as the point where the thermodynamic curvature ͑the curvature with respect to the thermodynamic metric͒ diverges. This is based on the fact that thermodynamic curvature is proportional to the correlation volume ͓5͔:where 2 is ...