"Any visitor to Gaza can't hold back his shock after seeing the substantial destruction," Mladenov told a news briefing. "I believe the Israeli blockade imposed on Gaza must be lifted."
Israel started a 50-day air and ground offensive on Gaza on July 8, 2014, leaving over 2,000 Palestinians dead, and substantial damage to housing and infrastructure.
The Bulgarian diplomat, crossing through the borders between Israel and the Gaza Strip Wednesday, toured the areas which witnessed significant destruction during the Israeli offensive last summer.
Mladenov stressed that reconstruction of the Gaza Strip must be comprehensive as "destruction... didn't only target buildings but also impacted humans."
Reconstruction efforts have been hampered by the Israeli blockade, imposed in June 2007 after Hamas seized control of the impoverished coastal enclave.
Israel, seeing Hamas as a hostile entity, says the blockade aims to prevent attacks from Gaza.
Over the years, Israel has eased the blockade in response to international pressure but never completely lifted it.
Meanwhile, Mladenov also urged the Palestinians to work to achieve internal reconciliation.
Feuds and profound disputes between Hamas, which still rules the Gaza Strip, and President Mahmoud Abbas' Fatah Party have been ongoing since Hamas's violent takeover of the Gaza Strip in the summer of 2007.
A senior Palestinian official, who spoke on condition of anonymity, told Xinhua that Mladenov held talks Wednesday with senior Hamas officials concerning the reconciliation and reconstruction of Gaza.