The X-ray observations of Cyg X-1, covering a period of 3.8 years, by the All Sky Monitor (ASM) on board the Rossi X-ray Timing Explorer (RXTE) were analysed to search for long periodicities. The periodicities in the original data were seen to be distorted by the high/soft state and the failed state counts of the system, and thus the power spectra showed two unrealistic peaks at 1608 and 406 days. The similar to 300 day modulation reported previously (e.g. Priedhorsky et al. 1983) is not confirmed by the present ASM data. Truncation of the high counts above 30 cnts/s partly eliminates the distortion and reveals the true period of 150(d) in the power spectra. It is interesting that the high/soft state (HS) and the failed state transitions (FST) always occur on the maximum phase of the 150(d) cycle.