A curable composition comprises (A) 10-30 wt.% polyfunctional cyanate ester, (B) 25-60 wt.% difunctional cyanate ester or its prepolymer, (C) 10-30 wt.% difunctional cyanate ester or its prepolymer, and (D) 5-25 wt.% thermally conductive filler comprising boron nitride. Component (A) has the formula: NCO-Ar1-R1-(Y-R1)n-Ar1-OCN (B) has the formula: NCO-Ar3-R3-Ar3-OCN (C) has the formula: NCO-Ar4-R5-Ar4-OCN wherein n is 1 or more; Y is R2-Ar2-OCN or Ar2-OCN; Ar1 to Ar4 is C5-30 aryl; R1, and R2: C1-3 aliphatic group or C3-20 cycloaliphatic group; R3 is a single bond or C1-2 aliphatic group; and R5 is C3-10 aliphatic group. Also claimed is a method of encapsulating component of electrical machine, which involves contacting the component with encapsulating material comprising curable composition.