By using a self-assembly method, SU-8 whispering gallery mode optical microcavities with an ultra-smooth surface (σ < 0.6 nm) and high-Q factors (∼10⁴) are fabricated. As an application of the microcavities, we demonstrate a polydimethylsiloxane packaged temperature sensor with high sensitivity (120 pm per °C) and long-term stability (over one year). These results illustrate broad application potential in ultrasensitive sensors and microlasers.