Feeding unsaturated fatty acids (FA) as Ca salts (CS) has been proposed as a way to protect ruminal microbes from the adverse effects of dietary oils. However, ruminal dissociation of CS may limit the efficiency of this protection. Industrial processes used to obtain CS of FA lead to the production of commercial feeds with a range of different particle sizes. We hypothesized that feeding CS as large particles will improve their inertness in the rumen and improve animal performance. CS of unsaturated FA were obtained from Virtus Nutrition LLC (Corcoran, CA). On a FA basis, the preparation contained 22.4% 18:1 n-9, 14.7% 18:2 n-6, 31.9% 18:3 n-3. The product was sieved through a 1.9-mm screen, and the retained particles were identified as coarse CS (CCS). The small particles were ground through a 0.864-mm sieve, and identified as fine CS (FCS). A mixture of unprotected FA, as triglycerides, with a composition similar to that of the CS served as control. Eight Holstein cows were used in a 4 × 4 Latin square design. Treatments were: 1) ruminal dosing of unprotected FA (negative control; N-CTL); 2) ruminal dosing of FCS; 3) ruminal dosing of CCS; and 4) abomasal dosing of unprotected FA (positive control; P-CTL). Treatments were adjusted to provide 600 g FA per day, and were offered daily in 2 equal boluses for 14 d, followed by 14-d washout intervals. Pre-planned contrasts were used to compare CCS with i) N-CTL; ii) FCS; and iii) P-CTL. Milk yield was 30.0 kg/d for CCS, and was similar in cows fed N-CTL (29.4 kg/d; P = 0.70) or FCS (29.6 kg/d; P = 0.82), and lower in cows fed P-CTL (25.8 kg/d; P = 0.01). Milk fat content was 3.48% for CCS, and was lower in milk from cows fed N-CTL (3.01%; P < 0.01) or FCS (3.15%; P = 0.03), and higher in milk from cows fed P-CTL (3.84%; P < 0.01). Concentrations of trans-10, cis-12 18:2 in milk was 0.41 mg/g of fat for CCS, and was higher for N-CTL (0.69 mg/g; P < 0.01) or FCS (0.69 mg/g; P < 0.01), and was similar for P-CTL (0.35 mg/g; P = 0.29). In conclusion, feeding CCS prevented ruminal production of trans-10, cis-12 18:2 and maintained a higher milk fat content as compared with unprotected FA or FCS.