A new series of Sm(CowFevCuxZry)z magnet products with 1Hc up to 13 kOe at 400°C has been developed. These magnets have low temperature coefficients of iHc and a straight line B vs. H (extrinsic) demagnetization curve up to 550°C. A straight line extrinsic curve provides greater design flexibility and facilitates reduced size and weight of magnetic circuits. The straight line extrinsic demagnetization B curves are required for magnets used in dynamic applications. The maximum use temperature, defined here as TM, can be determined by finding the maximum temperature at which the B curve is a straight line. The results of this work show that increasing the Co content in the magnet enhances high temperature performance. A higher Co content provides a lower temperature coefficient β of iHe. Magnets with Co content w = 0.65 have β = 0.26%C from 25°C to 300°C. Magnets with w = 0.82 have β = 0.116%C from 25°C to 300°C. The lower β results in a higher (BH)max at high temperature, and a higher TM. The TM is 240°C for w = 0.65,330°C for w = 0.68, 400°C for w = 0.76, 490°C for w = 0.78, and 550°C for w = 0.82. By increasing the Co content and adjusting the content of other elements, along with appropriate changes of processing parameters, a new series of magnets has been developed for use at temperatures from 400°C to 550°C. Magnetic properties vs. temperature for the new series of magnets, in comparison to those of the best commercial magnets, are shown in this paper to illustrate the improvements which have been made. Magnetic properties at different temperatures vs. Co content for the series are also reported in this paper.