Abstract: The effect on thickness noise of helicopter rotor caused by uneven blade spacing was analyzed. For hovering rotor with uneven blade spacing modulated in sinusoidal or cosinoidal mode, its thickness noise was calculated. Thickness noise calculation method is based on Formulation 1A which is the solution of FW-H equation. Compared with thickness noise for rotor with even blade spacing, the calculation results show that the magnitude of frequency spectrum lines on blade passing frequency and its harmonics are reduced and frequency spectrum shape is changed for rotor with uneven blade spacing when tip velocity of blade is subsonic. At the same time, the overall sound pressure level of thickness noise for uneven blade spacing is approximately same with that for even blade spacing. The result could be used to analyze the frequency spectrum change of thickness noise for rotor with uneven blade spacing.