What is the relation between pK_b and strength of base? 

The smaller the pK_b, the stronger the base 

The smaller the pK_b, the stronger the base. 

K_b is the base dissociation constant. The base dissociation constant is a measure of how completely a base dissociates into its component ions in water.

 pK_b = -log K_b. 

A large K_b value indicates the high level of dissociation of a strong base. A lower pK_b value indicates a stronger base.