T This the use of ground granulated blast-furnace slag (ggbfs) and coconut fiber has the potential to reduce cement consumption while improving concrete crack resistance. this study evaluated the compressive strength, splitting tensile strength, and ultrasonic pulse velocity (upv) of concrete with 30% ggbfs substitution and 0%, 0.25%, 0.50%, 0.75%, and 1.00% coconut fiber substitution by mass of cement. concrete without ggbfs and without fiber was used as a control. cylindrical test specimens with a diameter of 100 mm and a height of 200 mm were cured until 28 days. each mechanical result is the average of three test specimens, while the relationship between parameters was analyzed descriptively with pearson and spearman correlations according to the observation unit. the compressive strength of all mixtures was 36.71–48.07 mpa and exceeded the design quality of 25 mpa. substitution of 30% ggbfs without fiber reduced the compressive strength by 3.31% compared to the control. among the fiber mixtures, a 1.00% fiber content produced the highest compressive strength of 43.08 mpa. the splitting tensile strength ranged from 2.90 to 4.00 mpa; the mixture with 0.25% fiber provided the highest value, increasing by 38.10% over the control. the upv value ranged from 4,348 to 4,545 m/s and was categorized as good to excellent. the relationship between compressive strength and splitting tensile strength, as well as compressive strength and upv, showed a moderate negative correlation, but was not significant based on the number of observations available. the results indicate that mechanical response and internal quality are affected differently by fiber content; 0.25% fiber content was most effective on splitting tensile strength, while 1.00% provided a better compromise between compressive strength and upv.