Four new monoraphid diatoms are examined and described from a high-altitude (> 3000 m) region of western Sichuan, China. The descriptions are based on light and scanning electron microscopy observations, and the four new species are compared with similar taxa worldwide. Achnanthidium sichuanense sp. nov. has small, lanceolate-elliptical valves, with rounded apices. The outline and central area differentiate it from similar species. The unique characteristic of Eucocconeis undulatum sp. nov. is that it has valves that are undulate along the apical axis, forming an “H”-shaped area on both raphe and rapheless valves as seen in light microscope (LM). Platessa mugecuonesis sp. nov. and Platessa lanceolata sp. nov. are two small species, whose valve outlines and central areas make them different from other species in this genus. A. sichuanense sp. nov. and E. undulatum sp. nov. are discussed in terms of the relationship between “Achnanthidium & Psammothidium” and “Eucocconeis & Psammothidium”, as these genera are difficult to separate according to morphological characteristics.