We demonstrate a compact and high-performance 1 x 2 silicon MMI switch by thermo-optic control of symmetric interference modes using local and direct heating of the mode-peak regions. The direct heater is formed with n-i-n-i-n resistors whose i-regions are placed at the peak regions of the first two-folded image. Three electrodes are separately formed on each n-doped region for alternative heating of the two i-regions. Using a symmetric 1 x 2 MMI designed as a 50:50 splitter under no bias, path switching is attained by inducing a phase difference of +/-pi/2 between the two modes, applying a bias to either n-i-n heater. From this switching scheme, we achieve a low crosstalk of less than -30 dB with a compact active area of 3 mu m x 40 mu m. We also observe a fast switching response with a rising time of 1.04 mu s and a falling time of 0.89 mu s.