The single SHS (SSHS) ZrB2 powder was mixed with calculated amounts of H3BO3 and Mg and was subjected to a second pyrolysis with an attempt to convert the residual ZrO2 to ZrB2. The product of second pyrolysis is black in colour and consists of fragile porous mass with no lumps found. The XRD pattern of DSHS powder is shown in Fig. 1. The pattern shows that the intensity of ZrO2 peaks decreases as compared to SSHS powder. The wet chemical analysis shows increase of boron content and a decrease of oxygen content (Table 1). The unreacted ZrO2 content is again estimated, which is found to be ∼17 wt%. The decrease of ZrO2 content is an indication of increase of product conversion. The amount of ZrB2 and B51Zr are also found to increase in case of DSHS powder as compared to SSHS (Table 1). The HRTEM image of leached DSHS powder is shown in Fig. 3. The image shows formation of average particle of 25–40 nm in case of DSHS powder, which indicates decrease of particle size after double synthesis (Fig. 3a and c). The presence of ZrB2 is confirmed from the presence of (1 1 0) plane (Fig. 3b). During DSHS, the presence of existing ZrB2 may act as a diluent, which further decrease of particle size. The DSHS process is not only improve the product conversion but also decrease the particle size of ZrB2. We have also tried for third SHS but the SHS reaction could not proceed (no ignition), which indicates no need for third SHS to improve the product conversion. It can be say that the DSHS process is an important process for the synthesis of nanocrystalline ZrB2 powder with improved product composition.