We analyze entanglement properties of entangled coherent state (ECS), |

*α*, 0>

_{1,2}+|0,

*α* >

_{1,2}, with and without photon losses. By separating the coherent state into |

*α* >=

*c*_{0}|0>+

, we derive exact results of the logarithmic negativity

*E*_{N}, which quantifies the degree of entanglement between the two bosonic modes. Without particle losses,

*E*_{N}=1 for the N00N state; while for the ECS,

*E*_{N} increases from 0 to 1 as |

*α*|

^{2}→∞. In the presence of photon losses, we find that the ECS with large enough photon number is more robust than that of the N00N state. An optimal ECS is obtained by maximizing

*E*_{N} with respect to |

*α*|

^{2}.