Two field experiments were conducted during two the successive summer seasons (2007 and 2008) at the experimental farm of Sakha Agric. Res. station, Kafer El- Sheikh Governorate. The aim of this study was to investigate the effect of phosphorus fertilization levels  (0, 22.5, and 45 Kg P₂O₅/ fad for P₀ (control), P₁, and P₂) and potassium fertilizer levels ( 0 and 50 K₂O / fad for K₀ and K₁, respectively) on yield of two cotton cultivars; Giza 86 (CV₁) and Giza 88 (CV₂) and the soil availability of phosphorus and potassium during cropping seasons. The experiments were conducted in split- split plot design where cotton cultivars as main plots, phosphorus fertilizer levels arranged as sub plot and potassium fertilization levels as sub– sub plot, with three replicates. The obtained results can be summarized as following:  the yield of cotton was significantly affected by cultivars and fertilization levels  of P and K. Giza 88 attained higher yield compared to Giza 86. Application of P₁ treatment produced significant higher yield (26.0 and 24.8% in the two season respectively) than that of P₀ (control) treatment. there were about 6.8 and 7.6% increase in cotton yield in crop fertilized with P₂ compared to that with P₁ treatment.  Cotton yield was significantly increased by 13.3 and 12.5% in 2007 and 2008 seasons, respectively with K₁ treatment compared with control treatment (K₀). The maximum mean value of cotton yield were 11.7 and 11.5 Kentar/fad in 2007 and 2008 seasons were obtained with K₁ under P₁ and P₂treatments, respectively for Giza 88 varity. Application of P₁ increased the available P by 38, 45, and 37.7% in 2007 and 2008 seasons, respectively. Available P is declined with P₀, P₁, and P₂ treatments as function of the growth stage of cotton. The maximum mean value of available P (28.3 mg/ Kg soil) was obtained by application of K₀ under P₂ for CV₁ in April 2007 and 2008 seasons. the minimum mean values (8.0 and 7.5 mg/ Kg soil in September 2007 and 2008 seasons, respectively) were obtained by application of K₀ and K₁ under P₀  for CV₂ and CV₁ , respectively. Application of K₁ increased available K by (10.84 and 10.81% than that of K₀ (control) in 2007 and 2008 seasons, respectively. The equilibrium of available K in the soil solution  was re-established during cropping time.