Whey proteins are widely used as food material, but their functional properties are affected by processing conditions, in which Maillard reaction commonly occurs. The objective of this study was to investigate the effects of reaction conditions (saccharide rate (SR), setting time and setting temperature) on the functional properties of conjugates of whey protein isolates (WPI) - chitosan oligosaccharide (COS), which were produced by Maillard reaction. Response surface methodology (RSM) was used in this study, models for solubility, heat stability (HS), emulsifying activity (EA) and emulsifying stability (ES) of WPI-COS were established. Results indicated that the solubility of WPI-COS was 98.8 % at 29.8 % SR, 17.4 h, 46.7 °C; the heat stability of WPI-COS was 92.95 % at 27.2 % SR, 18.1 h, 48.7 °C; the EA of WPI-COS was 1.87 at 25.3 % SR, 20.0 h, 46.1 °C; the ES of WPI-COS was 13.28 min at 32.5 % SR, 12.0 h, 35.0 °C and setting temperature was the major factor affecting EA and ES while SR was the major factor affecting solubility and heat stability. Conditions were optimized for the preparation of WPI-COS to obtain improved functional properties.