This paper describes an experimental study that investigated the influence of ultrafine fly ash (UFFA), a relatively new pozzolanic admixture, on the early age stress development, stress relaxation, and cracking in restrained low water-to-binder ratio (w/b) mortars due to shrinkage. The restrained ring test was used to assess the early age residual stress development in mortar ring specimens. In addition, free shrinkage strain, elastic modulus, and splitting tensile strength measurements were performed to assess the stress relaxation (creep effect) and cracking potential. It was found that the restrained mortar mixtures containing the typical fly ash (FA) (ASTM Type-F) or the ultrafine fly ash demonstrated lower levels of residual tensile stress than the control mixture. Also, both typical fly ash mortar and ultrafine fly ash mortars demonstrated a reduction in free shrinkage. In addition, an increase in the age of cracking and a decrease in stress relaxation (creep effect) were observed in the restrained mortar ring specimens containing FA or UFFA. However, at equal replacement of cement with either FA or UFFA, the delay in the age of cracking and the reduction in stress relaxation were higher for the UFFA modified mortar.