This experiment was designed to evaluate the effects of the processing method of common vetch seed (CVS) (Vicia sativa) on laying performance, egg quality, metabolic parameters and liver histopatology during the peak production period in hens. Lohman layers, 46 wk of age in 6 replicate cages each containing 4 hens, were allocated randomly to one of four dietary treatments. Diets were control (C) diet containing no common vetch and experimental diets containing 25% raw common vetch (RCV), 25% soaked in water for 72 h with exchange of water every 24 h (SCV) and 25% soaked&boiled at $100^{circ}C$ for 30 minute common vetch (SBCV). Inclusion of RCV into the diet deteriorated all laying performance variables. SCV did not alleviate the adverse effect of raw common vetch on feed intake, egg weight, feed conversion, final weight and weight change. SCV partially alleviated egg production (p<0.001). SBCV diminished the adverse effect on feed intake, egg weight, feed conversion, final weight and weight change compared to raw vicia sativa (p<0.001). No significant difference was detected between SBCV and the control group in terms of egg production, feed conversion, final weight and weight change. Regardless of the processing method, all the common vetch groups had lower shell strength compared to the control group. Haugh units did differ between all groups (p<0.001). Inclusion of RCV and SCV into the basal diet decreased triglyceride, cholesterol, total protein and serum glucose concentrations (p<0.001). Hovewer, inclusion of SBCV into the basal diet increased these parameters. Liver samples were stained with Hematoxylin and eosin (HE) and evaluated by light microscopy. A biopsy of native liver tissue was used as a control. No histopathologic finding was present in the control group. Raw V. sativa compared with the control caused lipid accumulations in hepatocytes, severe congestion of hepatic blood vessels, inflammation, increased numbers of Kupffer cells and sinusoidal dilatations. Whereas, the livers from groups given treated V. sativa showed only different degrees of sinusoidal dilatations. Findings from the present study point out the risk of increased hepatic damage due to use of raw Vicia sativa. Increasing treatment of V. sativa lead to a decrease of liver damages. Inclusion of raw and soaked vetch seeds in rations affected adversely all parameters examined in laying hens. But alleviation was observed when soaked and boiled vetch seeds (SBCV) were fed. The results of these experiments indicated that soaked&boiled Vicia sativa seeds may safely be used at a 25% level in rations of laying hens.