Thermal expansion and magnetostriction measurements of the superconducting ferromagnet UGe2 under pressure were carried out. The temperature dependence of the thermal expansion coefficient shows a peak at the Curie temperature. When pressure is varied, the peak exhibits a maximum in the vicinity of a tricritical point (TCP), which separates the second-order phase transition from the first-order transition. From results of these measurements, we first construct the magnetic phase diagram including the TCP (PTCP ∼ 12.5 kbar). We also show that two lines characterizing the metamagnetism and the magnetic susceptibility emerge from the TCP. We argue that these magnetic properties in the vicinity of the TCP can be understood within a phenomenological frame of spin fluctuations.