  Let $V$ be a nonsingular projective 3-fold of general type. When the
pluricanonical section index $\delta(V)>12$, Chen-Chen \cite{Chen3} has a
complete list of the possibility for the weighted basket ${\mathbb B}(V)$.
However the possibility of ${\mathbb B}(V)$ could be infinite in the situation
$\delta(V)\leq 12$, which is the main challenge to the classification. In this
paper we mainly study the case with $\delta(V)=12$ and show that $Vol(V)\geq
\frac{31}{48048}$, which improves the corresponding result of Chen--Chen
\cite[Prop. 4.9(5)]{Chen3}.
