This work presents the theory and experiment of the wavelength continuously tunable lasers, whose external cavities are constructed by the microelectromechanical systems (MEMS) technology. The theoretical study reexamines the wavelength tuning theory and adapts it to the MEMS tunable lasers under special concerns of the system properties such as short cavity length and prealigned component position. It is shown that in the MEMS Littrow lasers a mode-hop-free wavelength tuning requires a moving pivot, or in other words, a fixed pivot has only limited tuning range. In addition, it is also found that the wavelength dependence of the gain medium refractive index significantly affects the wavelength tuning range. Based on the theoretical study, a tuning structure is fabricated by the deep reactive ion etching (DRIE) on a silicon-on-insulator (SOI) wafer, and hybridly integrated with a gain chip and an optical fiber. The laser has dimensions as small as 2.0 mm×1.5 mm×0.6 mm, and can be tuned continuously over 30.3 nm at a resolution of 0.03 nm/V2. As a comparison, another MEMS tunable laser designed according to the conventional theory has only a tuning range of 5.9 nm.