Calcified cyanobacteria and associated microfossils were examined in 8500 thin sections of Ordovician core samples from 64 wells in carbonate platforms of the Tarim Basin, Xinjiang Province, Northwest China. They include 32 species (including three uncertain species) belonging to 20 genera, most of which are from the Middle and Upper Ordovician. Two new genera and species, Acuasiphonoria ordovica gen. et sp. nov. and Gomphosiphon xinjiangensis gen. et sp. nov., and two new species, Proaulopora pachydermatica sp. nov. and Rothpletzella longita sp. nov., are described. Calcified cyanobacteria include Girvanella, Subtifloria, Razumovskia, Acuasiphonoria gen. nov., Hedstroemia, Cayeuxia, Bija, Apophoretella, Ortonella, Zonotrichites and Bevocastria. Probable calcified cyanobacteria include Proaulopora, Phacelophyton and Gomphosiphon gen. nov. Calcified Microproblematica include Renalcis, Izhella, Epiphyton, Wetheredella, Rothpletzella and Garwoodia. This assemblage is diverse in comparison with similar Ordovician fossils reported from other areas, and includes six genera and 14 species recorded from the Ordovician for the first time. Calcified cyanobacteria and their associated microfossils are more diverse in the Middle and Late Ordovician than was previously known.