The current work was done to study the impact of supplementation Spirulina platensis alage into the rations of growing Friesian calves on feed intake, rumen fermentation activity, digestibility, growth performance and economic efficiency. Eighteen Friesian calves aged 12 months with initial live body weight of 219.06 ± 5.35 kg were divided into three groups for 150 days feeding trial. Calves were individually fed a basal ration composed of 50% concentrate feed mixture (CFM) + 30% corn silage (CS) + 20% rice straw (RS) on dry matter basis without supplementation (control, R1) or with 1 g (R2) and 2 g (R3) dry Spirulina alage/ head/ day. Results showed that calves fed R3 recorded significantly (P<0.05) the highest nutrients digestibility and feeding values compared with R2 and the control one (R1). Intake of total dry matter (DM), digestible crude protein (DCP) and total digestible nutrients (TDN) were nearly similar for the different groups. The concentration TVFA's was significantly higher, while pH values and NH3-N concentration were significantly lower (P<0.05) in rumen liquor with the two Spirulina rations compared with the control one. Total protein concentration in plasma increased significantly (P<0.05) with Spirulina rations compared with the control one, while the rest of blood metabolities did not affected by the dietary treatments. Final live body weight, total and daily weight gain, as well as the improvement of daily gains were significantly higher (P<0.05) for the high level of dry Spirulina compared with the low level of dry Spirulina and also the control one. Feed conversion as DM, TDN and DCP required for producing one kg weight gain were lower significantly (P<0.05) for calves with Spirulina rations compared with those of control. Average feeding cost was nearly similar for the different groups. Whereas, feed cost per one kg live weight gain was significantly lower (P<0.05) for the two Spirulina rations than that of the control one. As well as, total revenue of body weight gain and net revenue as well as economic efficiency were significantly higher (P<0.05) for calves fed Spirulina rations than those of control ration.