One hundred and twenty five (125) cross and native cattle breeds were examined for prevalence of T. annulata infection for the first time in Siwa Oasis and evaluated ts effect on some blood constituents before and after treatment with buparvaquone. The prevalence of tropical theileriosis was 40.3 and 29.4 % in cross and native breed respectively using blood smear examination. Immunofluorescent antibody technique (IFAT) could identify T. annulata in 80.7 % of cross breed and 70.5 % of native cattle. In addition, there was seasonal variation in prevalence. The tick species H. anatolicum was recovered from 65.6% of examined cattle. Cattle clinically infected with T. annulata had significantly low levels of total proteins, albumin, magnesium, potassium and iron concentrations (P6 0.05) but AST, L 8 glutamyl transferase activities, total, direct and indirect bilirubin, creatinine levels were significantly high (P6 0.05). Buparvaquone was effective against both stages of T. annulata and succeed to control fever and temperature returned to normal range by 7th day post treatment. In addition, some serum elements returned to its normal values post treatment especially in native but not in cross breed cattle. In brief, our data showed that tropical theileriosis is prevalent in Siwa Oasis especially among cross breed cattle and the disease has some effects on hepatic and renal functions. There is a need for using immunization methods to reduce the losses from the disease.