Iodine value (IV) indicates the degree of unsaturation of milk fat, thus reflecting the presence of long chain unsaturated fatty acids (LCUFA), especially C18:1c. The higher the IV the greater the degree of unsaturation. Changes in dietary lipid content and composition can have a major effect on the IV of milk fat. The principal fatty acid in the forage component of the diet, namely grass or grass silage, is C18:3 (Murphy, 2000) while concentrate supplements contain varying proportions of C18:1 or C18:2 fatty acids. LCUFA undergo hydrolysis and biohydrogenation in the cow’s rumen producing mainly C18:0, the majority of which is converted to C18:1c by the desaturase enzyme systems, mainly in the mammary gland (Murphy, 2000). This experiment aimed to examine the relationship between milk fat IV and dietary lipid content, composition and diet type.