Dew is one of the most important moisture sources except rainfall for extremely water limited desert ecosystems and they have a significant effect on the maintenance of ecosystem structure, function and processes. To better understand the influence of biological crusts on dew deposition and evaporation processes in a semiarid sandy ecosystem, dew quantities of different surface types (bare sand, light algae crusts, dark algae crusts and moss crusts) were measured in the southern edge of the Mu Us sandy land in northern China using micro-lysimeters with a diameter of 7 cm and a height of 5 cm. The main conclusions were as follows: (1) There are 20 records in the experimental periods where dew was deposited almost every day except rainy days; (2) The total dew amounts of different soil surface types were (1.998±0.075), (2.326±0.083), (2.790±0.058) and (3.416±0.068) mm for bare sand, light algal crusts, dark algal crusts and moss crusts, respectively. Moss crusts were characterized by having the greatest amount of dew at dawn, whereas bare sand yielded the lowest amount of dew and the total dew amounts of biological crusts were significantly higher than that in bare sand (P < 0.05). Therefore it can be concluded that the formation of biological crusts benefited dew deposition. Dew quantities of biological soil crusts increased with crust development and the total dew amounts of different types of biological crusts varied significantly (P < 0.05). Daily average dew amounts of different soil surface types were (0.100±0.003), (0.116±0.004), (0.140±0.002) and (0.171±0.003) mm for the bare sand, light algal crusts, dark algal crusts and moss crusts, respectively. The maximum amount of dew for each type of crust studied was several times greater than the minimum and the daily average dew amounts of different soil surface types were significantly different from each other (P < 0.01); (3) Results of dew formation processes at 2 hour intervals indicated that dew formation begins at 19:00, inconspicuous dew was deposited from 23:00 to 1:00 which then proceeded from 1:00 to 7:00. From 7:00 to 7:30, continuous dew was deposited on the surface of dark algae crusts and moss crusts even after sunrise but not on light algae crusts. Results of dew evaporation processes at 30 min intervals indicated that dew evaporation begins at 7:30, and ends from 10:30 to 11:00. Dew duration time in bare sand and light algae crusts was significantly higher than that in dark algae crusts and moss crusts (P < 0.05). It can be concluded that well developed biological crusts may potentially enhance hydrological circulation in the upper sand layer in sandy land. (4) Dew formation processes can be affected by air temperature, land surface temperature, relative air humidity and temperature differences between the atmosphere and land surface. Dew formation processes are not simply linearly related to any meteorological factors. These data further the understanding of dew formation characteristics of biological crusts in semi-arid sandy areas and assist in evaluating the significant ecological roles of dew in semi-arid environments.