  In this article we study the blow-up phenomena for the solutions of the
semilinear Klein-Gordon equation $\Box_g \phi-m^2 \phi = -|\phi |^p $ with the
small mass $m \le n/2$ in de Sitter space-time with the metric $g$. We prove
that for every $p>1$ the large energy solution blows up, while for the small
energy solutions we give a borderline $p=p(m,n)$ for the global in time
existence. The consideration is based on the representation formulas for the
solution of the Cauchy problem and on some generalizations of the Kato's lemma.
