The latitudinal patterns of the optimal, minimal and maximal growth temperatures of phytoplankton are analyzed using linear mixed-effect models, and whether environmental temperature plays a role in affecting these thermal traits is tested. The optimal, minimal and maximal growth temperatures of phytoplankton decrease with latitude for marine taxa; whereas the minimal and maximal growth temperatures are relatively invariant with latitude for freshwater phytoplankton. The thermal breadth, defined as the range between the maximal and minimal growth temperature, is larger for freshwater than marine phytoplankton. In contrast to Jenzen's rule, there is no increasing trend of thermal breadth with increasing latitude. For most phytoplankton, the minimal growth temperatures are lower than the lowest environmental temperatures and the maximal growth temperatures are higher than the highest environmental temperatures. In marine phytoplankton, there is a strong phylogenetic signal in the minimal growth temperature and hence the thermal breadth. After controlling other variables (i.e. latitude and maximal growth rate) constant, the minimal growth temperatures of cyanobacteria and dinoflagellates are significantly higher than that of diatoms. The thermal breadths of cyanobacteria and dinoflagellates are narrower than of diatoms. The maximal growth rate is positively correlated with thermal breadth for marine but not freshwater phytoplankton.