In this study, magnetic metal nanoparticles (MNP) were prepared by using Fe, Co and Ni ions and NaBH4 as reducing agents in oil-in-water microemulsion system. The prepared MNP were used as catalyst in 4-Nitro phenol (4-NP) reduction to 4-Aminophenol (4-AP). It was found that magnetic Co MNP performed superior catalytic performances than Fe and Ni MNP for 4-NP reduction. The catalytic reduction experiments revealed that as the concentration of reducing agent, NaBH4 that was used in the preparation of Co MNP was increased, the complete conversion time for 4-NP to 4-AP was decreased as the sizes of Co MNP were obatined as 75 +/- 16, 60 +/- 9, and 37 +/- 6 nm when using 0.1, 0.2 and 0.4 M NaBH, respectively. Therefore, Co MNP prepared by using 0.4 M NaBH4 was used in the detailed investigation of various parameters such as the effect Co nanoparticles, reaction temperatures and time. The activation parameters of Co nanoparticle prepared by using 0.4 M NaBH4 as reducing agent was calculated as Ea = 32.1 +/- 3.1 kJ mot(-1), Delta H = 28.8 +/- 2.2 kJ mot(-1 )and Delta S = - 162.5 +/- 9.4 J mol(-1) K-1 that is the best catalytic performance amongst the other Co MNP5. The shelf life (storage) studies of Co nanoparticles showed that smaller Co MNP obtained by using 0.4 M NaBH4 reduce catalytic performance faster than the others, e.g, in the order of Co MNP(0.4 M NaBH4) > Co MNP(0.2 M NaBH4) > Co MNP(0.1 M NaBH4) in 20 day storage at ambient temperature. The activity% of Co MNP was decreased to 58 +/- 3% after five consecutives use while maintaining 100% conversion at every use corroborating the promising industry application of this magnetic Co MNP.