This paper focuses on the time constant for last passage percolation on complete graph. Let $G_n=([n],E_n)$ be the complete graph on vertex set $[n]=\{1,2,\ldots,n\}$, and i.i.d. sequence $\{X_e:e\in E_n\}$ be the passage times of edges. Denote by $W_n$ the largest passage time among all self-avoiding paths from 1 to $n$. First, it is proved that $W_n/n$ converges to constant $μ$, where $μ$ is called the time constant and coincides with the essential supremum of $X_e$. Second, when $μ<\infty$, it is proved that the deviation probability $P(W_n/n\leq μ-x)$ decays as fast as $e^{-Θ(n^2)}$, and as a corollary, an upper bound for the variance of $W_n$ is obtained. Finally, when $μ=\infty$, lower and upper bounds for $W_n/n$ are given.