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Silicon nitride (SiN) emerges as an important platform for ultralow loss photonic integrations with complementary metal-oxide-semiconductor compatibility. However, active devices, such as modulators, are difficult to realize on pure SiN due to the lack of any electro-optic (EO) properties of the material. Here, an SiN and lithium niobate (LN) heterogenous integration platform supporting high-performance EO modulators on SiN waveguide circuits is introduced. An efficient evanescent coupling structure is realized for low-loss light transitions between the SiN waveguide and the LN ridge waveguide with a measured mode transition loss of only 0.4 dB. Based on this heterogeneous platform, an EO Mach-Zender interference modulator on SiN is built with unprecedented loss, efficiency, and bandwidth performances. A half-wave voltage of 4.3 V with a modulation bandwidth of 37 GHz and an overall insertion loss of 1 dB is measured for a 7-mm long device. Data transmission up to 128 Gb s-1 with a bit-error-rate of <2.4 × 10-4 is also demonstrated. [ABSTRACT FROM AUTHOR]