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Let <inline-formula> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula> be a connected graph and <inline-formula> <tex-math notation="LaTeX">$d(\mu,\omega)$ </tex-math></inline-formula> be the distance between any two vertices of <inline-formula> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula>. The diameter of <inline-formula> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula> is denoted by <inline-formula> <tex-math notation="LaTeX">$diam(G)$ </tex-math></inline-formula> and is equal to <inline-formula> <tex-math notation="LaTeX">$\max \{d(\mu,\omega); \\ \mu,\omega \in G\}$ </tex-math></inline-formula>. The radio labeling (RL) for the graph <inline-formula> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula> is an injective function <inline-formula> <tex-math notation="LaTeX">$\digamma:V(G)\rightarrow N\cup \{0\}$ </tex-math></inline-formula> such that for any pair of vertices <inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\omega \,\,|\digamma (\mu)-\digamma (\omega)|\geq diam(G)-d(\mu,\omega)+1$ </tex-math></inline-formula>. The span of radio labeling is the largest number in <inline-formula> <tex-math notation="LaTeX">$\digamma (V)$ </tex-math></inline-formula>. The radio number of <inline-formula> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula>, denoted by <inline-formula> <tex-math notation="LaTeX">$rn(G)$ </tex-math></inline-formula> is the minimum span over all radio labeling of <inline-formula> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula>. In this paper, we determine radio number for the generalized Petersen graphs, <inline-formula> <tex-math notation="LaTeX">$P(n,2)$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$n=4k+2$ </tex-math></inline-formula>. Further the lower bound of radio number for <inline-formula> <tex-math notation="LaTeX">$P(n,2)$ </tex-math></inline-formula> when <inline-formula> <tex-math notation="LaTeX">$n=4k$ </tex-math></inline-formula> is determined.