We prepared bismuth sodium titanate (Bi0.5Na0.5TiO3) ultrafine powders by the sol-gel method. The dielectric properties of the pressed pellets and fired ceramics with different grain sizes as a function of temperature at various frequencies were studied. With decreasing grain size, the dielectric anomaly around 200°C increases, while the dielectric thermal hysteresis decreases. All the samples with grain sizes larger than 100 nm show dielectric peaks at temperature of about 350°C. The very little change in Tm observed down to the critical size indicates that Bi0.5Na0.5 TiO3 is an order-disorder system above 200°C. In addition, the dielectric peak becomes lower with decreasing grain size and the ferroelectric critical size of Bi0.5Na0.5 TiO3 was eventually determined to be about 100 nm according to the disappearance of dielectric peak.